Although ActivityPub does a lot to help share content across federated sites, it goes without saying it doesn't always go smoothly for a variety of reasons. Different post formatting, different overall focus and structure of other software, and so on.
With that in mind, it's as important to create different local accounts both on the same software (at times) and on other software to help polish up the content sharing.
As an immediately obvious example in the case of Lemmy: creating other accounts to help moderate a remote community due to issues of cross-site moderation.
However, it can go beyond that. Due to the minimal algorithmic design of a lot of federated software, many across these sites may not even be aware of some of the others. Some using Mastodon or a *key variant may have no idea of Lemmy, Mbin, or Piefed and vice versa, so may still be using corporate alternatives.
This means that it may also be beneficial for those trying to grow these respective communities to create accounts on Mastodon, *key, etc. sites and share links from Lemmy/Mbin/Piefed there, or vice versa. It's by no means ideal, but it's the reality of the situation at the moment.
Speaking of multiple accounts, it's important to try to correct misconceptions of only needing one account for this very reason.
For the moment, if you want to participate on federated forums, it's better to make an account on whichever site software supports it (like Lemmy/Mbin/Piefed), largely the same with federated microblogs (like Akkoma/*key/Mastodon), and so on.
Ultimately despite some of the inconveniences, taking this approach may at least be less of an uphill battle than navigating the increasingly closed corporate networks trying to keep people trapped there. Across the federated sites one may find more receptive audiences to checking out other open social networks that they may have completely overlooked.
p.s.
also maybe you're just a software sicko like me and you enjoy checking out other software from time to time and seeing what different approaches people are trying.
if you consider checking out microblogging i highly recommend the *key variants (e.g. Sharkey or the like) 'cause they're a much different vibe from the more Twitter-like stuff like Mastodon
