In the other world you have Facebook, YouTube and Instagram, you need three accounts, one for Facebook, one for YouTube and one for Instagram.
In the Fediverse it's the same, you need an account for the Fediverse equivalent of Facebook (Mastodon in your case), an account for the Fediverse equivalent of YouTube (PeerTube for instance) and an account for the Fediverse equivalent of Instagram (Pixelfed in your case).
What you get is that from Mastodon you can have friends on PeerTube and see their video appearing in your Mastodon timeline. It doesn't happen on Facebook: if a friend of yours post a video on YouTube you can't see that video from your Facebook account (unless a friend of yours on Facebook happens to see that video on YouTube and decides to share it on Facebook).
Another thing you get here is that you see ALL the posts of your friends appearing in your Mastodon timeline, without any algorithm deciding you can't see a post because it thinks that post doesn't deserve to appear in your timeline.
Another thing you get here is that you ONLY see your friends posts, you don't see posts of people you don't know just because the algorithm decides it's worth you read it.
Another thing you get is that you don't see ads.
There are some other advantages of this place but the most important, for me, are the ones I mentioned above.