Dad called this morning to talk filters. Here are some of mine. From left to right: Hoya circular polarizer, Hama UV, 7DayShop neutral grad. Here's how I use them...
Polarizer: I don't use this much. If I want to reduce reflections from non metalic surfaces, or increase saturation in landscapes. Can make ble sky look dark and evil. But most of the time it isn't necessary. And it robs a couple of stops of light, so you can't just leave it on full time. It's on a rotating mount so you turn the filter for best effect, and is a pain to use if your lens' filter thread rotates with focus.
UV: I rarely use this. I know lots of people use one of these as lens protection. I don't. I use a lens cap for that. I think they reduce contrast and can cause ghost images. I do use it in hostile environments. The beach on a windy day, boat trips, parties etc. If you do want to use one full time, pay the price, get a good multi coated one. Or at the very least a single coated one like the basic Hoya or Hama offerings and use a hood!!
Neutral grad. Great fun for darkening skies. I bough this cheap one from 7dayshop to play with. Like the polarizer it's on a rotating mount. I use it to darken overcast or cloud free skies. But these round ones limit you composition options. I have a square resin Cokin version too, giving the advantage of moving the grad up and down. But it's not very neutral, turning the sky green, so I hate it. Don't use it to darken skies with fluffy little clouds, unless you want fluffy little grey clouds. Use a polarizer for that.
Truth is I don't use filters much, but they're sometimes useful.