The main reason, for me, was that i got the actual price at the time of ordering, regardless of the price when paying (usually later, depending on the payment method).
Second reason was that the price at the broker got averaged and updated every 90 seconds, which allows to catch dips while the prices already goes up on exchanges already.
Something like an arbitrage-proximate time machine

Another bonus, whichever address the customer sends the Bitcoin to, if it's his exchange wallet, the original fiat buy is tainted.
And of course, you don't need to have money lingering in your exchange's fiat wallet to make a quick buy (preferably if bitcoin price goes south).
All of your above arguments are summed up in my first two: “KYC, or …”, but let’s not forget the hefty broker’s fee.
That said, it’s nice to see you posting something sincere for a change.
quid pro quo

OT: Got lost all the day in trying to fix my PV inverter after a firmware upgrade, when it refused to connect to the grid.
Support tickets, factory resets, analytics, research... no time to check BTC price

In the end, there are problems at the grid supply, the mains frequency is out of range most of the time. I have to call an insider i happen to know, he works as a tech for the power company. Seems related to the new firmware. I think it's a bug, because the inverter should limit export power, not refuse to sync to the grid at all.
I'm going to end this day with a few extra drops of my freshly filtered cannabis oil for recreational purposes

It has a ver different effect compared to vape and/or smoking, really warm and peaceful. That's probably why US cannabis-oil users often call their stuff "medicine".
However, nice to see King Daddy above $37k again, after the Binance/CZ story... Didn't do much harm to the market, all in all?