There are reasons why one would have multiple accounts... For example, one that's anonymous and one that can be linked to their real identity. Or an account that's coupled to one's brand and one personal account...

As for account sales: people could buy an account to level up faster... In the past this happened out in the open, everybody knew an account could change hands and one should always be careful dealing with anybody cause you never knew if the account was still owned by the same owner. Somewhere around 2015/2016 (IIRC), the community took a harsher stand against account sales, and even tough it's technically still allowed to sell accounts, the community usually tags sold accounts to warn other people they're not dealing with the original owner. Nowadays, account sales probably happen off-forum, and i personally think this makes people less vigilant. Personally, i don't care if you have two accounts or buy an account without malicious intent: if you don't scam, don't spam and treat me fairly, i couldn't care less if you're using a sold account or not... But yeah, sold accounts can be used to scam, and multi accounts can be used to join campaigns with multiple users at the same time, i certainly don't condone this behaviour.