Segments and Campaigns

I’m still confused about the relationship between Segments and Campaigns. I thought for awhile that contacts needed to be added to Segments in order to initiate Campaigns, but I also see that I can add contacts to Campaigns manually or as steps in other Campaigns… I guess I’m curious why segments exist as a whole step in the process. Is there any best practice reason to add a contact to a Segment that populates into a campaign rather than just adding to a Campaign first? (This is primarily for manual instances).

the only reason I’m seeing is I don’t see an easy way to search by campaign…

Can anybody help clarify?