for personal use, main reasons are you won’t have to worry about instance admins making arbitrary decisions that you don’t agree with, and no worries about server overload or downtime.
for making an instance for public, helping fediverse become a more viable alternative by spreading the load over more instances and helping it grow.
Right now the best way is to search from inside a lemmy instance itself. lemmy search finds much better results than what native reddit search used to give.
mentioned in FMHY’s unsafe list