I'm not sure which of the following GUEST meant

1 Why having changed the tuning to DADGAD form 'standard' chords. That is, why look for the same chords with different fingerings to make them sound the similar to chords in 'standard' tuning eg C chord (x32032) Am (x02232) F (303203) G (550450)? What would be the point making it sound the same as what you can do at least as easily in 'standard' tuning?

2 Why do a lot of similar chord shapes work? DADGAD is not really that different to standard tuning. You are dropping the top and bottom and fifth string notes by a tone. So - especially if you don't play every string in every chord (and I know people who do) - so some of your standard chord shapes become familiar sound - eg a C shape (x3201x) turns the chord into C7; an A chord turns into an A with a second which is a nice sound (x0222x - or with a second and fourth on top if you play it x02220 which is not discordant); a D chord (000232) turns into a rather nice sounding chord (played with the addition of one note it becomes a rather nice Dm9sus - 003232). But they don't have horrible discord in them so they work. A G chord is a little stranger as it has a 7th in the root position but becomes a G7 add2 and without the root note sounds less strange.
Now if you picked a tuning like EADGBD# a lot of things don't work! And it is an occasionally used tuning and does have a sense to it before anyone says 'but noone would tune to that'... Most different tunings make playing easier, not harder, and offer different opportunities and sound different.
If something is difficult to play (I have small hands) you can retune a string. I play in EADGCE when I play a Case of You because it works. I play a tune in CGDGAD because I wrote it like that and it doesn't work/I can't play it otherwise. And I sometimes play with a partial capo to give things I can't otherwise reach (eg capo at 2nd fret covering strings 1-5 gives a drop D. With 6th string tuned down to D and 5th tuned to G it gives the option of the fourth as a bass note on the 6th string). They are just solutions to problems sometimes rather than 'tunings'! With me it sometimes goes "I need x note on a string" and it isn't an option at the moment; if I detune or tune up that string can I play the rest of the song/tune? If yes then retune it.

3 There is another possibility. You might be Joni Mitchell. There are various explanations as to why she had so many different tunings. One very plausible one is that her childhood polio left her with a reduced ability to form chord shapes so, being a creative person, she found a different solution. ie She tuned the guitar around the chord shapes she could play. Brilliant! I hope that is a true story as I have read it in more than one source (eg Joni open tunings)