Depends on the soap.

Arko stick I use straight on my face or it can be pressed into a mug as its so pliable, both work. Some slice a bit off into a mug and use it that way.

Glycerine soaps like Col.Conk can be microwaved, tallow soaps can be grated then pressed into a mug/bowl (MWF, Tabac etc).

I'm a face latherer so I usually apply the soap either straight to my face or I load my brush then apply straight to my face. I don't like using a scuttle or a bowl to make lather in. Seems like wasted effort when you can be building a great lather on your face.