Glycerin can be used as a solvent when making transparent soap, although too much glycerin will make the soap "weepy" in humid weather. Small amounts could be added to regular bar soap, but again there is a risk that the soap will be weepy and too soft. Glycerin can be used to make liquid soap in place of or in addition to water.
But mostly glycerin is a byproduct of soap making, not an ingredient. When you make soap, it naturally contains roughly 8% glycerin by weight.
If the soap didn't already have some glycerin, it might be nice to add some. But it already has glycerin, so the benefits of adding even more are small. The disadvantages could easily outweigh the benefits.
The only times I use glycerin in soap is when I make shaving soap using stearic acid. I also use glycerin sometimes to make liquid soap paste.