I think the point is to build up a large list of accounts to cycle through that have existed for a while and seen sporadic legitimate looking use, rather than having to use a new account when suddenly starting to churn out spam.

A new account that immediately starts posting a lot is a big red flag. An old, active account that slowly ramps up posting volume is likely a lot harder to distinguish from a "real" user that is getting more active as the service gets bigger.

So you slowly build up a big portfolio of accounts and keep them "warm", and then spread your spamming over a larger volume of accounts instead of trying to create new ones and pump out lots of spam faster than they can block you.