The area below a window is exposed to more water than the surrounding areas of the masonry surface, because the large glass surface area of the window immediately sheds water to the smaller surface area of the brick veneer below; first collecting on top of the flat surface of the rollocks "sillbrick" before dissipating to the surface area below.

I recommend applying masonry sealer to this area every 5 years, provided the area is free of all voids, such as open mortar joints and/or spalling brick.

Excess Water Erodes Masonry:

Avoid future repairs by maintaining your masonry in its vulnerable areas where moisture is abundant.