#1
What purposes to different guitar tunings have? Like standard D tuning (DGCFAD) or Drop D (DADGBE)

I mean, If the song you already wanna play sound fine in standard tuning, why would you wanna play it in a different one?
#2
Different tunings facilitate easier playing of certain things. For example, Drop D allows you to play power chords on the lowest three strings with only one finger, as well as giving you they ability to easily play certain string skipping and arpeggio licks (the two strings are an octave apart, which allows for identical finger).

Lower tunings, such as D standard, C Standard, etc allow easier playing in certain keys. For example, whereas in standard tuning EM, GM, AM, and CM are very easy keys to play in (due to being able to use a number of open strings, allowing for open chords such as the EM and GM chords), in D you can play DM, FM, GM, and BbM fairly easily, since the lower tuning has open strings that fit more readily into those keys. Having a lower key makes it easier to accompany certain instruments (such BbM for a Bb clarinet) or to accommodate a vocalist's range (someone with a lower voice might have an easier time singing in FM than GM).

Of course, you can play in any key in any tuning, but certain styles that require certain heavy usage of open strings, such as country and bluegrass finger picking (the Chet Atkins/Jerry Reed tune Jerry's Breakdown is a wonderful example) or any of that sort of Andy McKee sort of playing, require open strings that are in whatever key you want to play in, so playing in the key of EbM in standard would be very tricky, since none of your open strings are in that key. This is also why we use capos for guitar playing (it works similarly to tuning up, but it's a lot easier to do.

Playing in lower tunings (like D Standard) is also popular because the lower tunings requires either looser tuned strings or heavy string guages, which changes the timbre of the instrument. For example, heavier strings are often described as having a "beefier sound". For this reason, lower tunings are particularly popular with metal, hardcore, modern rock, etc bands that want a "heavier" sound.

Similarly to this is the use of "open tunings" such as Open D (DADF#AD), Open E (same as Open D but a step up), Open G (DBDGBD), or Open A (same as Open G, but a step up), which allows you to both play certain chord voicings easily and allows you to have open notes in your key of choice (Open G is ideal for playing in GM). Certain instruments, such as the banjo and resonator guitars make use of these tunings for these reasons.

Another advantage of open tunings is that it makes playing with a slide easier due to the physical limitations of playing with a slide (you can generally only play notes in the same position, ie you can only play notes on the same "fret" at one time, unless you use certain techniques that are more commonly associated with steel guitar.

Speaking of steel guitar, there are also "extended" chord tunings such as C6, E7, E9, A6, etc, though these tunings are more common on steel guitar because they allow more closely spaced diads in a single position. That's probably best not to worry about, since it can be very confusing, especially since you probably aren't a steel guitar player and so you probably won't ever tune to E13 or B11. Though interestingly, the standard tuning for a guitar makes an Em11 chord, though it is almost never referred to as such.

There are also what are called "regular tunings" which have all the strings tuned to the same interval, with All Fourths (EADGCF) being the most common for guitar. These tunings allow you to use the same fingerings for chords and scales across all of the strings, whereas the Major 3rd between the G and B strings in standard tuning throws that off.

Other Regular tunings aren't common on guitar (and All 4ths isn't even common to begin with), though bass guitar can be considered All Fourths (and extended range basses, such as 6 and 7 string basses, have the higher strings go up in fourths as well (C and F). Also All Fifths tunings are used on other instruments, such as the mandolin and its relatives (mandola, octave mandolin, and mandola), violin and its relatives (viola, octave violin, and cello), and tenor banjo.

Some tunings are also used to replicate other instruments my using the same intervals. For example, Open G can be used to mimic the Russian guitar (tuned DGBDGBD), EADF#BE is used to approximate the the tuning of a lute (usually with a capo on the third fret to actually make it the same tuning), and New Standard (CGDAEG) shares its four lowest strings with the cello and mandocello and its 5th-2nd strings with the octave violin, octave mandolin, and Irish tenor banjo.

I'm sure that there are other things that I am not thinking of at the moment, but those are some of the main reasons.
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#3
/\ This /\

This guy's got you covered. There is soooo much more you can do with open tunings. So many different tones you can get (from the same chords you'd play in standard) and so many fun ways to play them!