Washes are just like paints, inks, or dry pigments - they can do a number of things (but not
everything), provided you know how to use them to achieve that end. Some of the more common uses:
-You can achieve general shading by simply slathering the wash over the desired part and letting it dry. It will naturally pool in recesses, providing deeper shading than on adjacent flats or raised details.
-You can use a "pin wash" to help visually separate panels by applying a thin line of the wash at joins and grooves. The wash will give a more subtle gradient than simply lining with paint, but not as even as an overall application (the benefit being that you won't have to repaint areas you want to leave as light as the underlying coat of paint).
-You can apply "reverse highlights" to add visual interest to flat armor plates by applying a heavy overall wash, then soaking up the center pooling with a paper towel wick (or a sufficiently large brush). This will leave a smooth gradient from the lighter center radiating out to darker edges - not terribly realistic, but rather pleasing to the eye.
-You can also use ready-made washes to apply thin glazes of color, since they're already quite dilute. If applied sparingly enough that they can't pool in the recesses, the mostly transparent coloration of thin wash will help tint undercoats, soften layer transitions, or add a touch of vibrancy to muted tones.
Honestly, I can't recommend picking a few up enough. Like drybrushing, their use is one skill in a larger set, not the answer to everything (no matter how many new painter's debut models are met with a resounding chorus of "put a wash on it" and nothing else

).
Get a pot of a color or two you will likely use (Badab Black and Devlan Mud are the most universally useful, I'd say, but I also use quite a bit of Thrakka Green on my Orks and wouldn't mind picking up a few other colors) and start playing around with them. Soon enough, you'll learn the common tricks and pitfalls (using gravity to control pooling, how to avoid ringing, etc.), or will have enough of a sense of them to benefit from asking more specific questions.
Best of luck!