@wrenkins using the pin isn't always feasible with some of the antique machines. If I use a modern spool on most of my machines in use, the thread wraps around the pin regardless of how slow I actually sew. I either have to rewind smaller spools or figure out another way.
I use this for my 201, and have the FW versions on those:
https://singer-featherweight.com/products/thread-stand-old-iron-for-singer-15-66-201-and-many-singer-clonesAt the studio, I have wall mounted stands...sort of a weird cross of industrial thread stands and thread racks since I can use them on my industrial treadle/treadles and domestic machines in use there without having to have different items for each...they all work.
My treadle at home uses a mug system because it's in my living room and not always set up. Kids, cats and a DH who think it's cool forces me to close it up. Well, it's not really a mug...it's in an antique string minder that comes out of the top with a spool pin inside...but why I use that is a different topic.
Now, I DO use cone thread because I am cheap for the most part, but there is times I have modern spools. Like now at home where I have been making masks. The Coates and Clark I just bought for it won't work on the treadle because it's cross wound, so I don't use the pin at all.
As to why the mug
@Sewot part of it is for stabilization, part is to keep the thread from turning into a mess and bouncing all over the place and part is to keep the thread path where you want it. I know myself, I sew too damn fast to just put it on the table behind. And yes, my treadle sews as fast as a modern one. I can get it to 2000 SPM and sew somewhere in the 750-1000 SPM range usually (unless I am sewing buttonholes, then it's a 1-2 count with the ticking of the attachment and the speed is dramatically lower).
edit: Pete, you did the same thing a mug/safety pin does with your set up.