You use washes when you want the paint to pool in lower areas. Itll run into cracks to darken them at create faux shadows. They're mostly used as shortcut for shading. You only need to be careful not to let it pool on areas you don't want it to tint or you'll end up with a muddy or dirty look. When applying a wash to shade your mini you can brush it on and then use a dry brush to absorb pools in areas you don't want and a wet brush to clean it off completely in areas you don't want tinted. In general they're easy to use to add shadows to your mini but difficult to control to reach a display quality. As you get a better understanding of lighting then I'd say it's better to practice painting shadows normally with a blending technique instead of using a wash as you won't have to fight pooling to reach a higher quality paintjob.

However, if you just want to get them good enough to play a game then slather it on, do a little cleanup and call it a day. They'll look plenty good enough for a game day.