In short, to protect you from wet weather and keep your leather clothes looking good for longer.
Leather is a natural material with open pores. Moisture can penetrate these pores, and dirt can accumulate in them. Impregnation forms an invisible protective layer on the leather. This allows moisture (for example, sweat) to escape from the inside, stops wetness penetrating from the outside, and means dirt can be easily wiped off. Leather shoes or leather clothing stay in good condition for longer and are easier to clean.