The best reason I've ever been told is because it doesn't sound spanish-y enough. Por ejemplo, vuelvo (I return) is beer than Volvo (a car). Another reason is because some words are the same as some word conjugations such as juego (I play/game) w/out the stem change is jugo (juice). However, it is mostly for the spanish sound, not the word difference.

These changes occur only with certain verbs as you realized. It´s because in spanish we have some irregular verbs and for conjugation their vowels change. It´s something you will learn with practice. :) Good luck.