Why decentralisation matters in social networking, from the Join mastodon blog. https://blog.joinmastodon.org/2018/12/why-does-decentralization-matter/
To which I would add that with a decentralised network, if you are not happy with the way your instance (independent server on the network) is operating, for example because you have found out they are selling user data, you have the option of moving to another server on the network without losing touch with all your contacts, as you would if you moved from one centralised network to another. This makes the network as a whole more resilient to bad actors.
Giving each instance responsibility for the behaviour of its users on the network also seems to me a reasonable compromise between free speech and protecting people from online abuse. If you feel that the moderation guidelines on your instance are too restrictive, you are free to move to another one. However if a given instance is consistently publishing abusive content, then other servers can block it from contacting their users. see: https://blog.joinmastodon.org/2018/07/cage-the-mastodon/
One of the first Mastodon instances, there is no specific topic we're into, just enjoy your time!