Primary reason to grinding content : To Gate Content.
Quite often, the creator has designs to allow both game mechanics and storyline to interweave, which forces players to be geared/leveled at an appropriate level. I don't think many people can suspend belief enough to see either Lisa/Loren, take on the last boss with a peashooter or a stick.
Secondary reason (sometimes the most important) reason to grinding : To tell a Story or explore the Game Mechanics in place.
Self explanatory but don't we all want to watch our hero/ine blossom into a beacon from the shroud of obscurity? Not to mention all the aspects of an RPG and the myriad of gaming ideals
Personally, I don't believe enemies should level at all. But grinding should be allowable to either allow easier access to content with time/equipment. Or to allow access to impossible content/self fulfilment. That is up to the creator's devices.
Anyone familiar with Fire Emblem will know that Apothesis requires 50+ hours of grinding to complete (with zero deaths).
I believe grinding also gives satisfaction as each minute is spent boosting your character/esteem, bit by bit