"Today I only eat side dishes."
Translation:오늘 저는 반찬만 먹어요.
It's kinda weird for an object in a sentence to have two particles or postpositions at the same time. So we had to drop 을 which is an object marking particle because 만 (only) is alreadt used. So, in this context it is appropriate to drop 을. Moreso, in some kind of way, 반찬 is obviously the object in that sentence, so adding 을 is not all that necessary.