I've just started, and everything says learn your hiragana and katakana first.
The hiragana, very clear. I guess you can say an awful lot of things in Japanese with hiragana. But everything I've been reading all seems to say that katakana are mostly used to form loanwords from other languages. I don't see how a beginner would need to use loan words early on.
So what is the importance of the katakana? I have absolutely no doubt I'm badly underestimating them.
But everything I've been reading all seems to say that katakana are mostly used to form loanwords from other languages
Katakana are used for way more than just making loanwords:
This is just off the top of my head, but katakana has many more uses than just foreign loan words.
I've just started, and everything says learn your hiragana and katakana first.
Yeah, start with basic things first.
I don't see how a beginner would need to use loan words early on.
Won't you ever write your own name, even though you're a beginner?
So what is the importance of the katakana?
As one of the three common ways of writing Japanese, and not an incidental one, it is very important. It's not based on frequency of usage (although they are more frequent than you seem to think), but on getting the foundations of the language, right from the beginning.
Your question sounds a bit like "why learn capital letters? They appear a ridiculously small amount of times compared to lower case letters!"
English does not have a special designation for loan words. Given that there is no demarcation of loan words in the language, it's almost certain that you're simply unaware of loan words you use every day.
The distinction between "derived from a foreign word" and "loan word" is a distinction without a difference. Nearly all katakana-written loan words are adapted in both meaning and pronunciation in Japanese. To be more accurate, "loan word" is the term used by Japanese learners to refer to "words derived from foreign (non-Chinese) languages". The Japanese language happens to have a handy way of demarcating these words, specifically spelling them in katakana.
A huge number of these "loan words" are currently displacing the Japanese-origin and Chinese-origin words in both casual and technical language. I won't say you can't get by without learning them, but it would be like only studying the most common capital letters in English... you're going to regularly find yourself effectively illiterate.
Some terminology: Hiragana and katakana together (as opposed to kanji) are called "kana". They may have used "hiragana and katakana" so as not to bombard you with new terminology. Also, "gairaigo" is a Japanese term for loanwords from western countries.
I don't know an awful lot about English. I'm merely a native speaker of it. So I can't really comment about loanwords in English, except to quote the following from James Nicoll:
The problem with defending the purity of the English language is that English is about as pure as a cribhouse whore. We don't just borrow words; on occasion, English has pursued other languages down alleyways to beat them unconscious and rifle their pockets for new vocabulary.
But there are some words in Japanese that don't have a non-gairaigo form because they were invented in European countries. As an example Is タオル used for the towels used at onsen? had the following as an answer:
Towels were introduced in the Meiji era and were almost exclusively imported from England. That's why the word タオル came from English, and there is no other word for them (although Japan of course had their own fabrics before that (I'm specifically talking about terrycloth)).
Regarding "getting by". While in beginner textbooks there's no kanji, a lot of hiragana and a lesser amount of katakana, in my experience of Japan as a tourist, there's a lot of kanji, a fair amount of katakana, and a small amount of words made up entirely of hiragana. There's some words made up of kanji plus hiragana, but knowing only the hiragana part won't help you much.
The main time I've noticed hiragana in Japan are words like です and ください and さん. I didn't notice many nouns, or many verbs being written entirely in hiragana, though I could be mistaken.
A major benefit of learning hiragana is to help with learning how to pronounce Japanese. That's because the relationship between how something is written and how it is pronounced is straightforward for Japanese written in kana. The problem with Romaji is that you have to remember how "a", "e", "i", "o" and "u" are pronounced in the context of Japanese, as opposed to in English.
You have a logical jump from "learn first" (what textbook says) to "important", and actually, your inference is wrong. Katakana is not particularly important than other letters (perhaps you mean kanji). Eventually, you would have to learn all. The reason you should learn kanas first is because they are easier and is a clear (established) set than the kanjis.
First, you don't "say things in hiragana", you say in Japanese. At most you can write things in it, but you can do it with katakana as well! Japanese writing have two distinct parts:
Think of those two variants as CAPS and lower case in Latin alphabet. Should you learn one or other? Well, sure, in modern writing you see more lower letters, unless you're reading titles or advertisement, but correct answer is: learn both and order doesn't matter much.
Actually you may even want to follow same pattern often used while learning other alphabets: learn hiragana and katakana symbol for same syllable at same time. Them often being similar helps too.
Here's the real reason: シャーロット・ランプリング. When your Japanese is really good and you're living in Japan, and reading Japanese newspapers for fun - you will know what they're saying but not who they're saying it about. I still remember us saying シャーロット・ランプリング over and over in various ways until suddenly one of us got it. Of course now it seems blatantly obvious.
