Why are they used?
Filters are used to improve image quality by altering the properties of light that enter the camera lens. They can be used individually or as combinations to affect contrast, sharpness, highlights, reflections, colours, colour balance, and light intensity.
(Coatings) applied to filters (and lenses) enhance light transmission and reduce reflections that can lead to undesirable aberrations in an image. In general, light transmission improves as more layers of coating are applied. A multi-coated filter is more effective than a single coated filter, which is far more effective than a filter with no coating at all. Filters are especially beneficial for older cameras with lenses that weren’t coated.
A filter is often used to protect the front lens element from dust, scratches and other damage that could lead to costly repairs. A filter is much easier and cheaper to replace than a front element and vintage cameras found with a filter attached generally have well preserved lenses. Be aware that a cheap filter may have a negative effect on the optical performance of a finely-tuned coated lens.