Privacy. Whenever you spend from that single UTXO, however you’re paying will see your change. Also, if anyone pays you and then sees that payment get consolidated together with the rest of your coins, then they’ll have an idea of how much bitcoin you have.
Discussion
Is there a specific reasoning for the mention of 20% of your stack for a max UTXO? I definitely went too big on some recent cold storage transactions and it seems like it makes more sense to move funds in sizes closer to an optimal UTXO based on expected transaction fees