I don't use subaccounts, i don't think it is really important for a lot of traders, particularly those who have a small portfolio. Subaccounts could be necessary for big time traders who have a lot of strategies and need to monitor them independently, other than that i don't see the need to set up a subaccount within an exchange. Neither do i like using multiple exchanges, because of the need to submit KYC in all of them.
That depends on the OPs trading situation. Sometimes, people are too organized, they want subaccounts as they have specific use case. So that will be their personal choice whether to create subs or not. But if you are just a small trader and not very active in trading, I don't think you need subaccounts. Just the main one is more than enough.
I don't se any reason having subaccounts because it's with the same exchange and your funds will be at risk if anything happens to the exchange which is the main reason why splitting of funds into different exchanges is the best if you want to use multiple accounts to open different trades. However, I can understand with some people who says that a particular exchange gives them luck to make profit when trading compared to the other. Is this even true or a superstitious belief.