One of the most powerful tools in the musician's arsenal. Particularly in today's click-laden, quantized world. There's the obvious function: it keeps perfect time and gives you something to play along with. Less obvious: it helps you focus. For best results: pick an exercise, set a timer for a few minutes and play along with the click. Seriously, try it. Effective. Even meditative.
Once you've tried it, get an app! Easier to deal with. Lots of them out there.
Other instrumentalists learn scales, drummers learn rudiments. Little exercises for your hands. They help you play with finesse. This site is the go-to because it offers video instructions, practice routines and playalong tracks for all 40, as well as a suggested order of operations.
Timeless. Many would say essential. $12. Will last you the rest of your life.