Light pollution is an unwanted signal in astrophotography. Light pollution can cause gradients, soften images, and weaken the background vs object contrast. With an LP filter, you can block some of the traditional "man-made" signals (streetlights) and gain contrast in your imaging by filtering out the unwanted LP signal. In addition to blocking this unwanted LP signal, you can often image longer subs to gather more detail and object signal. With heavy pollution, you saturate your sensor quickly, but with an LP filter, you can take longer subs and collect more data.