Manual caching downloads the area of your choice saving having to stream it even if you’re online or offline. Just means you wont have to stream in the areas you have choosen to download ( saving bandwith ) I manual cached the whole of the isle of white while also being online, I fly over the isle of white alot so this saves me having to use this in the rolling cache - flying over the area means i dont have to stream it which improves performance. i have good internet so the difference isnt that noticable for me. good option is you can cache in high detail which means flying over those saved areas at high altitude looks fantastic. If your internet isn’t that good, you can choose those areas you fly over alot and manual cache them for offline as well as online 
the only issue is high quality can take up alot of space eventually.
https://forums.flightsimulator.com/t/how-to-create-and-manage-manual-cache-regions/136740