Before I move forward and explain you what is alternate tuning, I would like to warn you that I am no expert when it comes to alternate tunings. But I do know that what I know about alternate tuning is sufficient enough to teach people.
There is plenty of alternate tunings, and some of it I know you are familiar with an example is drop D tuning and modal D tuning or what is popularly known as DADGAD. But the question is why do we need to do alternate tunings? The answer to that is this: some songs are more comfortable to play with on certain tuning especially when the chords of the song need a certain open chord or open notes. But of course, it is always dependent to the song creator. It’s up to him, if he created the song using a certain tuning, well, people have to play it like that if they want to cover it or it would never sound right.
An example is the song “You Stand Here with me” by Creed. It is on drop D. the reason to this is they want to hit a certain low note, lower than E. Drop D allows them to have a low Eb and Low D.