The most noticeable form of foam is foam floating on the stock surface. It is easy to monitor and relatively easy to handle and is more a cosmetic issue.  Surface foam may cause problems with liquid levels and give overflow leading to pools of oils around the equipment which is a safety concern.
Additionally, this might reduce the process speed and availability of process equipment. The main mechanical problem tends to be when foam enters the system as air is a poor lubricant, meaning metal to metal contact can occur.