Its the confidence, across a lot of different product lines, that you are getting the right thing that will work first time. I've built my own dashboard parts from actual carbon fibre, spec'd out HIDs for an old car (projector lights, so safe) and so on. I have done some 3d printing stuff for the 3, but when I want a wrap, roof seal, interior LED's and some sill covers, being able to go to one place that lots of people have good experience's with who say 'these things work, fit well, have the right connections and don't blow your electrics' is really valuable to me.
That everything will come twice round the world does suck tho
. Wish they had a UK or EU shipping centre. Most items are even on UK amazon, but they all have a max of 8 reviews with 3x 'NEVER BUY THIS, IT DIDN'T FIT/SCRATCHED SOMETHING/ATE MY GERBIL', 3x 'perfect fit, and it gave me CPR when I needed it' obviously fake reviews and a couple that are not of the right product. Again, just going to someone with a reputation is worth it.
Maybe I'm just getting risk averse in my old age. Used to do my own tuning, now look at me