The main reason for using a wetting agent is to allow the substrate to be re-wet easily if it becomes dried out.
 

This can happen due to long delivery times and different growing techniques. The speed of wetting is very important as it is the absorption speed that ensures a more uniform water uptake.
 

This is shown below by the PBG test at Naaldwijk, Holland in 2000. In the top figure you can see the results for German black peat and below the results for the speed of water uptake by capillary for Baltic white peat.