If you copy this folder, you will copy all of your projects. But this is way more than you need to copy.
Even if you are using an external harddrive, you can use “Perforce” to let UE4 copy the files to your harddrive automatically. You can tell it what files and folders it should copy and it will create versions so you can go back to an earlier version if you broke something.
https://docs.unrealengine.com/latest/INT/Engine/UI/SourceControl/index.html
Perforce is for free if you only use it for <= 20 people. You can find tutorials about setting it up easily in the interwebs or ask for help here/on the forum.
I guess the most important folders of a project are the source, content, saved and config folders.