If you do regular check in, you have to wait in line - and it can be a longish line. Then the castmember has to check all your info, get your charging to the room info, explain how everything works - opening doors, charging with magic bands for example. Plus info on transportation, park hours, EMH, lifeguard hours at the pool, yada yada. Doing all this with everyone in line takes time, but is necessary with newbies.
Online check in gathers all the info they need to create your account and is designed for repeat visitors that don't need all the info. You just pick up a packet with basic info and a map to your room and go. You can still ask questions if needed, but info won't be given unless you ask for it.
Streamlined check in is the same as online but all you get is your room number. Show your ID to prove you're the right person, and get a room number and be on your way. If you need to pick up Magic Bands, or ask a ton of questions, or a first timer, don't use this option.
First timers should use regular check in. First time visitors to THAT resort, might want to.