What's the most important tool every musician needs to own? Well, if you want to play consistently and mesh well with other musicians—if you want to set toes tapping and heads bobbing, you need a metronome. Practicing with that steady "tick-tock" will keep you solidly grounded in time and help you learn how to play both with and around the beat, all invaluable skills for any player.
Since its invention in 1815, practicing with a metronome has been criticized by some as leading to "mechanical" playing, but using one with proper practice techniques will vastly improve your time sense and ability to play "in the pocket." Many metronome techniques exist to enable musicians to internalize time, learn to play consistently ahead of or behind the beat for specific feels, and to properly learn rhythmic division and polyrhythms.
Ranging from old-school, wind-up clockwork, wooden pyramids to modern digital units with programmable beats, there are metronomes to meet every need and budget, from simple practice to rhythm coaching and live click tracks. There are even wireless units that sync to each other and vibrate silently so the whole band can groove in unison.