Why are Kanji characters used along with Hirangana script characters for a word? For example: 飲みますwhich is spelled as nomimasu (correct me if I'm wrong) uses a kanji character. Why can't の be used here?
It's mostly a matter of custom, but the big reason is that using kanji helps convey meaning. This can be particularly helpful in cases where a word may have homophones, or be a less commonly used word, so having that semantic aid can be quite important. The effect is even more prominent in the Chinese languages, where all characters are based on meaning.
For example, if you saw the sentence 「かみをきる」 (kami o kiru) then you have several homophones to worry about. When we say かみ are we talking about paper (紙), or hair (髪), or a god (神)? Is きる talking about something being cut (usually 切る, although there are actually a few other kanji with similar reading and meaning) or worn (着る)?
Alternatively, if you saw a word you're unfamiliar with but it's written in kanji you recognise, you can take a guess at the word's meaning based on those kanji. In extreme cases, you might even be able to guess the meaning of a kanji based on its radicals (e.g. lots of kanji for different kinds of fish use the same fish radical - 鮪, 鮭, 鮫, etc.).
Kanji also make it easier to see how text is broken up. Most Japanese writing uses very little punctuation, and rarely put spaces between words, so a paragraph written in kana (hiragana and katakana) can actually be a lot harder to read than the equivalent including relevant kanji since you can tell from a glance where the words are separated (mostly - when you get to things like newspapers and very technical writing, the kanji density can get so high that it's harder to split words accurately again without re-reading the text multiple times). At a glance, you might not be able to see where the individual words are in a sentence like たなかさんからかったほんをもうよまなかった, but once you have practice with kanji you can look at 田中さんから買った本をもう読まなかった and clearly see some of the main breakpoints like the verb at the end, the object in the middle and the relative clause at the start.
And, of course, since most Japanese people start formally learning kanji quite early in their education, if you write solely in kana then it will tend to look more childish, although as a foreigner you would probably be given the benefit of the doubt in such cases.