Honestly, it could be any and everything. Sometimes, I choose herbs because I'm inspired by a certain colour or colours. (Like, I will legit say I want a red mask or a green mask today. If I could get blue spirulina I'd have blue days as well.) Sometimes, I'll choose herbs because my hair needs something in particular, like moisture or strengthening or a bit of extra cleansing. Sometimes, I'll choose a combo because I want to create online content. I also try different herbs or combinations because I know other people haven't (en masse) (e.g. goats milk powder). I just like to create.
I'm currently developing a certain routine where I'm being more intentional about the kinds of herbs and when I use them. It still gives me room to create new recipies, but I can narrow down which part of my stash I'm going to dip into.
As for yucca root: I started using yucca after hunting down some non-shampoo/herbal options that are in use by other cultures. Yucca was easy enough for me to get, cost effective, easy to make and makes for a super fast wash day outside of the time it takes to boil the water and let it steep. I gave up clay for about a year, in favour of the yucca. Turned DH onto it as well and I swear it was arresting and maybe even healing his thinning crown. But then I wanted to try ayurveda again and decided that I could mix my herbs into the clay for one stop shopping. I hate doing two steps -- so no cleansing clay mask, THEN ayurvedic mask, when I can just do them both at the same time.
With this is mind, I am now designing my masks to perform certain duties, but I think that I'll use yucca once per month, because, who wouldn't want a 10 minute wash day?
Side note, I find that yucca leaves my daughter's hair feeling so soft after. She's got a different texture to mine. I'm sure that it's doing the same thing for me, but on days where I have a lot of frizz, the texture of the frizz hides the softness. Hand in hair right now, I only used moisturizer once after washing and my hair feels light and airy. Time to add some moisture so that I don't dry it out.