As important as it is to learn how to play drums properly, it is equally as important to tune drums right and to take the time to understand drum tuning.
Imagine if you will, your guitar player walking into the studio every evening for rehearsals and just picking up his or her guitar, plugging in and just beginning playing This never happens, well it should not, because over night strings go out of tune and the guitar will sound very bad.
So your guitarist will always fine tune theirs first before beginning rehearsals.
The same holds true for drum heads, the heads or skins on your kit react to change in temperature and as the skin on your body will constrict in cold weather and loosen in warm weather, your heads will injure the same harshness from day to day if stored in attics, garages or indeed anywhere that is not a climate controlled environment.
And let's face it, none of us have that much money that we can afford to have a room with climate control for our drums.
Another reason for drums going out of tune is simply from playing them.
When you tune your set, you are seating the skin down on the bearing edge of the drum shell, however over time of the skin being battered by your sticks it will shift and lose tension, this will cause the drum to sound deeper and eventually will loose its resonance and sound dead, so its important to listen out for any change in pitch as you are playing and adjust slightly at the end of the song you are playing, to bring the brightness back into the drum.
Understanding how easily your kit can fall out of tune is so important if you want to be taken seriously as a drummer.