Why decentralisation matters in social networking, from the Join mastodon blog. https://blog.joinmastodon.org/2018/12/why-does-decentralization-matter/
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!