Droppers get the seat out of the way when you want to move around standing on the pedals.
You should do 1x and the dropper at the same time. The lever takes the place of your front derailleur pull lever, and the cable routing goes about the same way (assuming bottom pull derailleur). You will need an external routed dropper. Most of them now are internal or “stealth”
How much travel you can use depends on your fit. You can get an overall max length for you, and compare it to what’s published for your post. It might be blocked from full depth by a bottle boss especially on a medium frame.
https://pnwcomponents.zendesk.com/hc...me-and-height-
Your bike has kind of a mast at the seat tube and not much post showing so I’m guessing you will be doing 125 mm or 100. It’s possible to cut the mast down if you are ambitious.
Nearly all droppers are zero setback so consider that for the fit.