It decreases the life of the battery. Some older battery technologies actually needed to be drained nearly completely to get a full charge into them when recharging. Today's lithium ion batteries are just the opposite. The lower you drain it, the shorter the life of the battery, i.e. the sooner it will get to the point where it won't take a full charge.
In general, don't use the battery unless you have to.
There are power settings, and these settings take effect for different power configurations. Usually there's a battery profile that puts things(including the cpu) into a slow power saving mode. This may not be what you want, and you can change these settings, but the results may be that the battery drains a lot faster as it is expending more power.