227132
It's a phonetic thing.
The "e" in Japanese is pronounced like a hard A. Well, not exactly, but that's a basic way to describe it. Though Pocket Monsters has a soft "e", Pokemon is technically short not for "Pocket Monsters", but for "Poketto Monsutasu" (possibly both), with diacriticals about, yadda yadda. They're the same thing, but...not. Point is, in "Poketto Monstasu", the "e" uses Japanese phonetics, not English ones, so by adding an accent, we get closer to the Japanese phonetics.
It's not perfect, though. We still say the "e" and the second "o" in a more pronounced manner. Some people pronounce the "e" as "eh", others as "ay", when really it should be in between the two, but it's not really a big deal. Plus, it's pretty nonsensical since the second "o" isn't given a diacritical- so as to sound like "monster", but the whole reason any of this holds water is because the first "o" is pronounced as , well, "O", which doesn't make grammatical sense since the "e" isn't silent and the second "o" is pronounced differently.
tl;dr, it's to make it sound both like the Japanese pronunciation and an English marriage between "Pocket" and "Monster", but the word makes no damn sense.