It's obvious that the sound of the kanji character is 'ta' but why use a kanji character when you could've used た ?
Yeah, it's way confusing. Techinically, you wouldn't need to use kanji. But, it's just how the language is written. As I understand it, when the Japanese borrowed kanji for their writing. They took kanji for their sounds. Thus, if the ancient Japanese word for eat was 'taberu', they had no way to write that yet, so they got a kanji that had the 'ta' sound. Since the ta always stays constant in the word no matter how you conjugate it, they assigned a kanji to it. But since you have to change the 'ru' sound a lot in order to conjugate, it would be kind of hard to have many specific complex kanji for each conjugation of る. Thus they used simplified kanji (i.e. hiragana) that were easy to write for conjugation. This doesn't really explain why べ is written in hiragana. My guess is that since べ is "outside" the kanji, it helps denote that the verb is Class 2 and helps you figure out how to conjugate it (as opposed to a word like 帰る, which, though it sounds like a class 2 verb, we know is a class 1 verb because the え hiragana is "inside" the kanji).
Also, kanji ultimately do make reading A LOT easier. With no spaces and little puncuation in writing, it helps you separate words. When hiragana is used for conjugation, you get familiar with what is possible, so you know the hiragana is probably conjugation when right after kanji. It is very hard to read when it is pure hiragana.
'Course, this doesn't help that words are still sometimes completely written in hiragana. I have talked to Japanese friends who don't like to overuse kanji because it looks too Chinese. Thus, you'll see most onomatopoiea (spelling?) words written only in hiragana, because, I guess, they are "pure" Japanese. Also, they seem to throw in words written in hiragana even when there are kanji available.
Just think of this like English spelling. English spelling is ridiculous. It would be so much easier if it were phonetic. Yet, it is the way we do things and it does make things easier, in a way.
Don't worry about this too much. It will all fall into place.
Good luck.