This is soldering flux that has powdered solder in it. You apply it to
the joint, heat, & apply solder. Just like you do for regular flux. So
why use C-flux?

It's not pre-tinning, as that means wetting the joint surfaces before
assembly.

And if the C-flux isn't applied to the whole joint, the whole joint
isn't going to be wetted, just like regular solder.

The Rector Seal web site says "When heated, it reduces the surface
tension in the joint and allows the solder to flow easily and uniformly
into the joint." Which is what regular solder does.

What am I missing?

Thanks,
Bob