I started DIY mixing when I got back into my own colour processing after an 8 year hiatus.
4 years in a small apartment when first married saving towards a house and the darkroom gear all stuffed into a storage closet, instead of being used in the basement darkroom of a shared house I had owned with two other guys.

Then 4 years living in Australia with all my darkroom gear left packed up in my parent basement.
Finally back in Canada, but now with two young kids as part of the fold.

I knew from prior forays into colour in the old ep/2 world that the chemistry kit in a box route to printing on your own meant mixing the kit and using it over a few weeks of printing like mad before the chemistry pooped out.

So I ordered a starting batch of raw chemistry stuff from Johnnie at JD Photochem.

Then I contunued to built up and broaden my stock with orders from Mike at Artcraft, and some Formualry orders, and later Claire when she took over from Johnnie.
I do dearly miss the great service JD offered me.

With raw chems at hand:
I could mix a litre or half a litre of c-41 and process the films at hand, with fresh chems to suit the backlog.
I could mix a litre of RA-4, and process in tube, and know that if I did not finish the litre, the left overs poured away at the end of a day or two only were a small loss, not $40 or more like kit quanities gone off of the past.

Only when a roller processor landed in my lap for what I consider a great deal did I look to to buying kit based Kodak RA-RT chemistry.
My home brew RA developer oxidized in no time in the roller tank.

I have also mixed from scratch for E-6, and also use pre mixed 6 step E-6 from the cast offs of a photo processor who abandon slide processing.

By then I then found I had the lion's share of what was needed to diy mix my b&w chemistry of all sorts as well.
Some times I will 'dip a toe in' to try a mix, and then order the whole kit from the formulary. The try before a big buy is what I like with DIY mixing.

It is also nice to know that if I will be away from the darkroom ( I have a busy professional and family life), DIY gets me on my feet in almost no time.
It is just a matter of one night to mix fresh chems and the next night I am priniting or processing a backlog of films with chems that I know have not gone dodgy from sitting unused , or worse yet, partially used for some months. Too many past brown liquid in part used bottles of Ilford Multigrade developer no more.

I do have lots of commercial mixes that land on me as I buy up darkroom lots , and use them when still vailble.

I do a lot of cooking food from scratch, put up preserves for winter, make my own soaps from lye and fats, and also make some of our own wine and vodka, so diy photo chemistry is just one more thing to mix up to put into usable form.

Alt process curiousity is another factor.
If I am sizing and coating my own paper, then why not diy formulate my own developer to suit the needs of the project at hand?