I know y do it , but why !

BFL13 wrote:
There is also the difference between the stranded type with the thicker wire strands, and the type with a kabillion silky thin strands.

Can't say which has which, but they have cords that are especially flexible in freezing temps. I find the silky ones are very hard to use in a set-screw terminal, as they keep pulling out. Need much "strain relief".

I think one type is for "welding" wire, but I have no idea why.

Good topic! Should learn something I think.

The more strands in a given size wire the more flexible it is. The wire with hundreds of very fine wire is often called "welding cable" because that application typically calls for a high degree of flexibility so the welder can more easily manipulate the welding tools. The fact remains though that a 10 AWG wire will have exactly the same total metal cross sectional area and total circumference whether there's one strand or a thousand. The physical diameter of the multi-strand wire will be a bit larger than the single strand due to the air spaces between the strands, but both will have exactly the same 30 amp ampacity rating.