I only have one sub-account and it hasn't been used for a long time. Initially, the purpose was to differentiate between the two, as one was purely for trading and the other for long-term asset storage.
At the time, I didn't understand the dangers of storing coins long-term on exchanges.
However, once I understood, I only stored them in my wallet and the sub-accounts were no longer used.
I also have several accounts on different exchanges, but KYC forced me to abandon some accounts on exchanges and only use one account on my local exchange, there is no reason to open an account on another exchange because the local exchange alone is enough to meet my trading needs.