Standard guitar tuning is
EADGBE (those letters are the tones for the strings low to high.)
The alternate tuning for Iris is very unusual and I don't think it's a good example to introduce alternate tunings.
One very common alternate tuning is "drop D" where the low
E string is dropped down a full step to
D. That tuning is common for both classical and rock styles.
Another kind of alternate tuning is "open" tuning where the strings are tuned to various chords. A common open tuning is "open G tuning"
DGDGBD. Notice that all those tones produce a
G major chord.
Doesn't it make it more difficult to play?
In the sense that standard tuning chord shapes won't work and you need to learn new techniques then "yes" it will be more difficult to play.
But in some ways it can be easier, or at least not that hard to understand.
Open G tuning is popular for playing the blues. Basic 12 bar blues is of course based on chords
I IV V. Playing those chords is super easy in open G tuning. Just play all the open strings for
I, barre across the whole neck at the 5th fret for
IV and at the 7th fret for
Typical blues licks add various tones to those basic chords. One way to conceptualize those added tones is alterations from the main chord tones. The minor seventh is a full step below the chord root, the minor third is a half step below the chord third, a flat fifth is a half step below the chord fifth, etc. etc. It's fairly easy to create blues licks by simply stepping up/down from those basic chord tones. This explanation is a bit wordy, but the actual performance process is pretty easy.
...why would you use them?
In the case of playing slide blues in open G tuning - as one example - it produces a "sound" that you can't get with standard tuning. If you want that particular sound, you really need to use the alternate tuning.
Also, alternate tunings are used because they provide a different timbre for the guitar. This is fairly subtle, but it is noticeable. Many open tunings lower the strings and that results is a kind of deep, twangy tone.
If you play a song which uses a unique alternate tuning, and you really want to get the sound right, you will need to use that unique tuning. But that's not really about standard guitar technique and very specific to particular songs. The fingerings used for such songs won't apply to other tunings.
I think a versatile guitarist should have some familiarity with common alternate tunings. But for casual playing, like strumming basic song chords, you don't need to know how to play in alternate tunings.