It’s more transparent, visible, and allows stakers to hold validators accountable since their stake plays a material role in these rewards. With side deals, they could be doing this and stakers would have no idea. Also means validators have more revenue they make which might incentivize them to lower commission without losing income.
It also leads nicely into this proposal: SIMD-0123: Block Fee Distribution by jstarry · Pull Request #123 · solana-foundation/solana-improvement-documents · GitHub