It's not mandatory but recommended for better operational efficiency. For most of the users, using the same account as stash and controller is normal. But for power users and entities using multisigs or hardware wallets, having the multisig/hardware wallet account as the stash account and the maintainer's account as the controller account could save a significant amount of time and effort (in case of reallocation of nominations or payout claiming or validator maintenance).

Separating roles in your accounts can provide a better understanding of which transactions are being performed by whom. This can also enhance the security of your wallets by disconnecting the stash and making them 'colder.' Additionally, it allows you to authorize a controller to perform specific actions. While it is not mandatory, it is highly recommended to consider this approach.