The main account is relevant when you have Auto-Select enabled with your card. If you pay a category that is not matched by any sub-accounts, the money gets deducted from this „main“ account. And you cannot configure this one. And it is not the sub-account where you pay your plan from or receive your interest.
I have had some trouble with that, because I have created the account for me personally first and created a joint account with my wife as a second sub-account. I wanted the joint account to be my „main“ account for Auto-Select. But there is no way to change this. bunq support confirmed that this is always the first account. They told me I could remove my personal account to make the joint account my next first account, which I did in the end.
So, there is a case for a main account.