What is a service patch

A server patch is whenever they fix bugs, do maintenance on the server, etc. Why do they do it? So that the servers are running appropriately for us to play on. It can also add things to the game like new raids, dungeons, quests, anything they want to put in the game in small portions is put into it by a patch. Whenever they want to drastically change it is when they release expansions. You do not have to pay for patches but you do have to pay for expansions.