I’m both SPO and one of the owners. For me, the main reason to have a reward account is for easy award counting and distribution. Without a separate reward account, I would have to use my own private account to receive reward. If my private wallet is also receiving and sending transactions for other purposes, the reward accounting will become very messy pretty soon.
Also to my surprise, I recently discovered that the staking reward for all owners, in addition to pool reward (cost+margin), also go to the reward account. That increases the complexity of accounting and also require higher trust among owners and SPO. A separate reward account just makes accounting/distribution more transparent and easier.