What is the reason for having a soldered drain cock?

[Doug Horrigan]

The majority of joints on my central heating system are threaded, apart from a few that are push fit.
However, the drain cock is a soldered fitment, so the question is - why is this soldered (unless on some older BSP systems) when a threaded joint is suitable for everything else?
Just seems bonkers to me and makes a frozen drain cock far harder to replace.

 

[snowhead]

On a traditional copper pipe heating system most joints are soldered.
To use a solder drain off it only needs a copper end feed / capilliary fitting tee.
To use a threaded drain off it needs a brass 15/15/1/2" (or 1/2"/15/15) tee, both cost more than soldered and take slightly longer to fit.

 

[ShaunCorbs]

Also that’s not good if most of your joints a compression