I started modding drones around four years ago when I got the original Mavic Pro, try a few searches in Modifications and you’ll find all sorts of guides that various members here have put together over the years on how to mod anything from the DJI app itself, to a Crystal Sky, to a Spark, right up to an Inspire.
Back then it wasn’t as easy as it is now, the point-and-click apps like NLD (also search here for that too) and Drone Hacks didn’t come along until a couple of years later.
I’ve always enjoyed tinkering with stuff so that’s half the reason I do it.
The other half is because of DJI and their fucking stupid restrictions, their NFZ maps have absolutely no bearing on actual UK Flight Restriction Zones (again, many many posts here on that subject). Lots of people, myself included, are pissed off with DJI preventing us from taking off where we are actually legally allowed to take off.
The other half (there’s a lot of halves) is to enable FCC power mode rather than using CE power mode. In a lot of urban areas the DJI signals go to shit because of local wifi interference. Enabling FCC mode gives you that power boost to cut through the noise and maintain a strong signal in an urban environment, even at 500m away.
Beyond those “main” settings that people tweak then are the Parameters (again, search through Modifications ) and this is where the real fun starts, seeing just how much you can squeeze out of your drone. Why? Because there’s a leader board in place!
Beyond speed, you’ve things like ascent and decent rates that can be changed, 101 other settings are buried in the parameters.
Then there’s the rolling back of firmware. DJI are notorious for putting out updates that either contain bugs, or worse, will ground your drone unless you agree to, or do, certain things. They once (in)famously grounded the Spark until you installed a forced firmware update.
The list goes on man… 
Again, Drone Hacks as an app is very new, the DJI modding scene has been around forever 
The Autel modding scene is relatively quiet, simply because Autel don’t place these restrictions on their drones to start with.
If you don’t mind, I’ll tweak your topic title here slightly, as this thread will probably be wider than just a single app 