Also in the early days content federation was very poor (it still is sometimes) and multiple accounts were a necessity because if the instance federation was broken or someone wasn’t subscribed to that community on your end it just didn’t exist, was inaccessible, or was completely empty. So it makes sense to have multiple accounts.

Also instances do die. Many of the instances that started up when Lemmy first became popular aren’t around anymore. Big players still are but the really small no-name ones like (i.e Lemmy.ninja, unilem.org, etc.) don’t exist anymore. Having multiple accounts safeguards against that by having redundant ones that will still be there when and if the others disappear.