Skirts were believed to be effective against HEAT, and in some circumstances they were--in some they weren't. Angle, calibre, and several other factors had to be taken into account, of course, just like a "hull-down position" was often--but far from always--advantageous. While it did help against standard armour-piercing rounds, as well as ATR ammo, that was a matter of course--any additional armour is going to help against these. For the guns the Panthers and Tigers were facing, the extra armor wasn't all that important. APCR and the like were also disturbed by skirts, as it could cause a tumbling effect--good against people, bad against armor.

Wooden skirts occurred but were never doctrine for any military, just like sandbags. Most places you'll read will tell you that their effect was mostly psychological, although having to pass through extra mass will reduce velocity (a little), causing reduced penetration (a little).