Exactly what’s the topic says , what’s the reason behind it? Is it for easy mating or?

Two reasons I know of:
Some hyper active dog breeds can't control their long tail, they easily get injuries on the tail or they find it disturbing and annoying sometimes.

Other reason I know, it's because of the looks of the dog and to create a consistent identity/feature. They look tougher and firm without the long wagging tail.

The rotweiler, cane corso etc. Are known for docked tails.

While the Caucasian shepherds etc. Are known for cropped ears with similar reasons

For Fashion
Breeds like the rottweiler, bullmastiff, boerboel are more attractive when their tail is docked

d two major reason are....1 balance on seating position... 2 Bueti....it add more flavor to d dog look...

It's because those dogs you see with cut tails were used in the past to pull loads and the tails were cut to prevent them from getting cut in the machinery they were pulling e.g. Rottweiler in ww1