Last night I had dinner in a ramen restaurant in northern Japan and was surprised to read the katakana "ライス" (raisu) on the menu. This is obviously the English word "rice" borrowed. But what kind of rice or method of preparation might it refer to given that Japanese already has "kome", "gohan", and "meshi"?
ご飯 (ごはん), 飯 (めし) and ライス all refer to the same thing: steamed rice. ご飯 and 飯 can mean meal, too.
As you said, it is not uncommon to see ライス in a menu at a restaurant, even when it is not part of a compound word such as カレーライス. I do not know why they do not say ご飯, and I can only make a guess at possible reasons:
- As Jeshii said, they may want to make it sound like something fancy by using a loanword instead of the more common word ご飯. (But I am not sure if calling it ライス really sounds fancy compared to calling it ご飯.)
- As Uronym said, they may serve steamed rice on a plate, in which case it is understandable to call it differently from the usual steamed rice in Japanese cuisine, which is served in a bowl. (But the question is about a ramen restaurant, and I would be surprised if a ramen restaurant in Japan serves steamed rice on a plate. The use of the word ライス is not uncommon in ramen restaurants.)
- Depending on context, ご飯 refers to meal, and they may want to avoid possible confusion caused by this usage. (But some restaurants do write ご飯 to mean steamed rice and there is no possibility of confusion. Note that “set meal” at a restaurant has a separate word 定食 (ていしょく).)
As you can see, I am not satisfied with any of these reasons. If there is a better explanation, I am happy to learn it.
ライス is used for non-Japanese rice dishes, I believe, like curry or rice served on a plate in Western fashion. カタカナ and borrowed words are also used as 'fancy' or 'elegant' alternatives in Japan, especially in advertising.
You know how Eskimos have 80 words for snow?
Seriously though, I think it has to do with the fact that there are certain dishes that are western in origin. They use rice, but they are served differently. Take a look at dishes like 「カレーライス」 or 「ハヤシライス」 or 「タコライス」. All of these are served on plate or with western ingredients. ご飯 and 丼 are usually served in their own dish or in a bowl with more standard things on top.
There's also the fact that カタカナ英語 is very common. For some anecdotal evidence, there was a tour book for Tokyo Disneyland that I bought once that literally had combinations like, 「ファンタスティックスな素晴らしさ」. Basically they were just throwing in カタカナ英語 to enhance it and make it look special.
You'll find lots of words where there is カタカナ英語, a "native" Japanese word, and an imported 漢字 compound all for the exact same thing. Japanese is a language that does this a lot.
In addition to the other answers, ライス is sometimes simply used for disambiguation. The meaning of ご飯 gohan largely overlaps with "meal", so it can become ambiguous whether you're talking about a "meal" (as opposed to à la carte) or "rice". The more specific words for "rice" like 白米 can be too specific, so ライス is a broad, convenient descriptor.