There are two kinds of musicians: ones who use metronomes, and ones who don't realize they need a metronome.
The ability to keep an absolutely steady beat is learned like any other skill. It's tough at first, because we are human beings and not machines. I've had many students chafe against metronome work because it brings to light the tiny variances of tempo that they didn't notice before, and feeling like you're getting worse instead of better is frustrating.
Spend some quality time with your metronome and you'll find yourself improving in the following areas:
Complex rhythms: All rhythms are relative to the tempo you choose. No matter how fast or slow a piece is played, a half note lasts twice as long as a quarter note. For this to be true, there needs to be an established beat that all other rhythms reference. This becomes even more important when the rhythms you play are complicated; in order for the music to make sense, the listener must be able to feel the underlying, steady pulse. An uncertain, halting tempo is jarring to listen to and makes any sort of rhythmic complexity impossible to communicate.
Rubato: The art of "robbing time" is a zero-sum game. True rubato doesn't add or subtract any time from a measure; it simply borrows time from one note and adds it to another. You still reach the end of the measure at the exact same moment you would have had you played everything as written, with nothing shortened or lengthened. This skill is especially important when you collaborate with other musicians; they can only accompany you well if they can predict when you'll reach the end of the measure. You should never make your collaborators guess.
Accelerando and ritardando: A gradual (and intentional) increase or decrease in tempo only makes sense if the tempo has been clearly established. Otherwise, it just sounds like you aren't sure where the beat is.
Long story short: in order to do any sort of higher-level playing, you need a strong sense of the beat.
How to use a metronome:
Don't let it lead you. Meet the beat. Pretend it's a pianist chording along with you or a tuba playing quarter notes.
Pay attention. If you can't hear the metronome over the sound of your instrument, use headphones or hook the metronome up to some speakers.
Start with a slow, easy tempo. If you can't get your downbeats to line up with the beats of the metronome, slow down the metronome until they do align.
Put away your instrument! Sing or clap the rhythm along with the metronome. Take away the distractions of articulation, fingerings, and posture so that you can focus more fully on the accuracy of your rhythm.
Subdivide. Using a metronome to indicate the quarter-note beat is useful and is the most common method for tempo work, but see what happens when the tick of the metronome indicates eighth or sixteenth notes instead. Challenge yourself to be accurate down to the smallest subdivision possible!
Don't be deterred by expense; there are many online metronomes available for little to no cost. Metronome Online is my favorite (works best on laptops/PCs), but there are lots to choose from. Pick your favorite and start the new year with amazing rhythm!