Every time you make a transaction you reveal the size of an utxo you own. You probably don't want to reveal your full stash to every random stranger you transact with throughout your life.

A practical example: you want pay the plumber with Bitcoin. If you pay with a utxo worth several millions now that dude, that knows you and has been at your place, knows you have that much. He talks about it over a beer with some friend, who happens to be not so hardworking and nice guy. The friend decides to pay you a visit and whack the crap out of you until you give him some serious money.

This is a exaggerated (or not) example, but the reasoning is still valid. There's some privacy and security concerns around having your whole stack in a single utxo.