Useful translates better to שימושי. It comes from the word שימוש that has the meaning of usage. להשתמש meanse "to use". תועלת is " benifit". In any case, meaning wise both tanslation are ok.
I always confuse תועלת with תולעת, and that's going to get me in trouble one day :(
In hebrew words have roots. Usually of three letters. It is not directly evident (more on that latet) but the root of "תועלת" is "י,ע,ל" that also generate words like יעילות, התיעלות, מתיעלים. The point made is that the order of the letters of the root stays the same in all the words derived from that root. So the 'ע' is always to the right of the 'ל' in this root. The readon why there is no 'י' in תועלת although it is part of the root comes from pronounciation comfort that over the years removed it.