What paint stops damp coming through walls?

Anti-condensation paint contains insulating materials which improve the thermal performance of surfaces it's applied to. It can reduce wetness on walls and associated mould if condensation was caused by a cold surface. Anti-mould paint contains fungicides which kill mould spores and prevent their spread.

How effective is bitumen waterproofing?

Bitumen is a black, sticky substance with physical characteristics not unlike that of the molasses at room temperature. As is typical with hydrocarbons, bitumen is highly hydrophobic in that they tend to repel or at the very least don't mix well with water, hence their use as a material in waterproofing.