We know what can be saved to a file, but what should we save? Writing data to a file uses disk space. Therefore, we want to save only what we need to. It is better to think, "Which information do I want to keep after the server shuts down?" For example, a banking plugin will want to keep the balance of each account. As another example, a PvP arena plugin will not care about having information about an arena match. It is quite likely that the match will simply be canceled as the server is shutting down. When considering the teleportation plugin, we will want to still have the locations of each warp after the server is shut down.
Our next concern is when to save this information. Writing data to files has the potential to lag a server if it is a large amount of data. If you are unfamiliar with the term "lag", it is a phrase that is used to indicate that the server is running slowly. You know when this happens because the game becomes very choppy and players and mobs...