If you are going to be painting new or old plaster, or maybe a wall that has plenty of absorption such as a cement rendered wall, you may well benefit from first painting the surface with a pva sealer. The reasons for first applying a pva sealer are listed below.
Less paint used - When a pva sealer is applied to new or old plaster or any surface that has the ability to absorb water the surface will become sealed once dry. This will allow for a more even covering of the paint once applied to the surface you are painting. One area of plaster/surface will not absorb more paint than another area. Which leads to blotchy paint surface. The number of coats of paint required to paint the surface should be cut allowing for less work and less paint.
Adhesion to the plaster – when plaster or a cement rendered wall is painted, the paint often becomes flaky and peels of the wall, this is either because damp has penetrated the wall from behind or the paint has not adhered to the surface. Using a pva 
sealer first can help the paint stick to the surface better. Pva is a glue like liquid that is also used in glueing wood together and has many other uses also.
More even finish – On most surfaces that are painted there is different rates of absorption by the paint, some areas absorb a lot of paint and some hardly absorb any at all. This can lead to a blotchy surface that needs several extra coats of paint to cover the area evenly. By first applying a PVA sealer, all the surface plaster becomes sealed, so when you apply the paint there is a more even coverage of the paint.
Cost – Pva sealers are good value for money because they are, in most uses, diluted with water. This gives it exellent coverage of area, 5 litres of pva sealer, once diluted can give up to 50 litres of product based on a dilution rate of 10-1, which is very commonly used for a sealing solution using PVA. If you end up using 50 litres of pva, I’m sure its going to be cheaper than 50 litres of paint.