What's the use of that thing with addition and subtraction on the front fork of mountain bike? My oil fork.

Mine is XC28? The addition and subtraction on the left is to adjust the travel of the fork, that is, how much the fork can be pressed down (generally the adjustable range is 200mm), the lock on the right is to lock the fork, and there is a turtle and rabbit on the lower right to adjust the rebound speed. Basically all oil forks are like this. (The black one on the left controls the stroke of the preload fork by adding and subtracting rotation, and the above picture will be more intuitive. )