Over time, deposits and corrosion build up inside your instrument. This can impair valve or slide action, affect the sound and the "blow" of the instrument, and eventually damage the metal itself. With a chemical cleaning, I bathe the instrument in a special solution that dissolves the deposits without damaging the metal. A good way to check for corrosion is to look into your leadpipe. With a trumpet, you can pull off the main tuning slide and peer through the leadpipe toward a light source. With a trombone, take off the outer slide and do the same. With instruments that have curved leadpipes, you can try shining a flashlight down the mouthpiece receiver. If you see a rough surface inside with a lot of "gunk" clinging to the inside of the tube rather than a nice shiny surface, even after you have cleaned out the instrument with a snake or swab, it might benefit from a chem cleaning. The leadpipe and first branch of the instrument tend to get the dirtiest because they are the closest to where you are blowing, but the valves and valve slides can also get quite a bit of build-up. Below is a before and after picture of tuba valve slides that were chem-cleaned:

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Here are trumpet valves before and after being chem-cleaned and lightly polished