The general idea is that a desktop environment provides you with common graphical user interface elements such as icons, toolbars, wallpaper, and desktop widgets. In other terms it’s purely an aesthetic question. You can also decide which of these features will be useful to you and install the appropriate package(s) if you don’t want to grab the bundle that comes with any DE.

As far of timing is concerned, you can always experiment and install what you need as you go. The only downside to wait will depend on how good your distro is in managing packages dependencies.

Personaly, my Linux journey made me realize that the features offer by a DE were actually negatively impacting my productivity and a windows manager (a tiling one for me) was all I needed. But this decision - like a lot of others - comes down to personal tastes. Note that not using a DE doesn’t mean relying on the terminal only.