One of the two filters that I always carry is a circular polarizer. It’s great at removing glare and reflection from water, windows and leaves. I use a polarizer during the day when it has the greatest affect, I avoid it during early morning and evening as it removes a stop of light or so.
Polarizers add quite a bit of contrast to a photo, which is especially helpful in the middle of the day when the sun is high and the light is flat. They can also help for black and white photos.