When you tune to Drop D, you extend its range a full step lower. Tuning to Drop D makes it easier to shift your guitar to a range that makes it easier for singers with lower voices to hit the correct notes as you play. Drop D tuning also makes it easier to play certain riffs and power chords.

Why is drop D tuning so popular?

Dropping to D also has the advantage of dropping, so it's reducing tension on the string. Tuning down: safer than tuning up. It's also at the extreme low end of what a guitar can play, so you get an extra note, and one finger (or slide) power chords can be desirable.

Is Drop D tuning cheating?

No, it isn't cheating. I usually use drop D when i need to reach some tones that are impossible in standard tuning. Also, its quite useful in doing fast riffs. Now if you only play in Drop D, you are lazy.