With lemmy account you can only join "groups", so other Lemmy communities, or equivalent piefed or nodebb.

With a Mastodon account you can also join groups (Lemmy etc), but you can also follow single accounts from any mastodon, pixelfed, peertube etc instance.

Why then having a Lemmy account? The interface, how you see the posts, depends on the software. Lemmy is reddit-like, mastodon is twitter-like etc

If you want to upload your video to a peertube instance, then you need an account on that instance