I just think its a fun thing to do that some people are totally into (others not, which is ok too). My mom was a sewer and we have a couple family easter pictures with us in coordinating handmade Easter dresses... so cute and so dorky! There were 4 of us girls at the time, and my then 16 y/o sister is in this sexy off the shoulder 80's version of Easter dress, the 12 year old in matching 80's open shirt of the same pattern and an 80's belt, 6 year old me in a cute kid dress, and the baby girl in a little romper. All the same floral pattern. I know my mom must have been so proud of her ability to knock out four matching ensembles.
This same mom totally encouraged the twins' in their love of matching "same but different" clothes (these two were #5 and #6 of the family so that was beyond my moms sewing capacity, thankfully for all us older sisters who were mostly teenagers and up by then) but my mom bought them everything in coordinating sameness, like identicle blue and green dresses, and they loved it. They were doing "same but different" well into 4th grade.
All 5 of my sisters and I dress pretty similarly though and swap clothes when we can.
I haven't done too much matchy-matchy for my own girls, mostly because I rarely purchase them clothes I have so many hand-me-downs. The few times I have it is pretty darn cute.
I also remember a period where my elementary BFF and I would buy each other outfits for our birthdays, and show up to the party wearing that very outfit- like my new outfit? Got it for you too! Then we would call each other and wear it to school on the same day. We loved the attention and when people asked "did you plan it?" we always said no :)
I guess I have to admit I am pretty down with the matching, for funsies.