Synchronisation is the process where a generator is synchronised with the electricity network. That is desirable for two reasons:
a) An individual/company can use synchronisation to connect to the electricity network in order to share the load of his facilities, between a generator and the electricity network. It can, also, act as a maximum demand shaper for the electricity of the facility. In this way, you will get part of the power of the main electricity and afterwards, when the charges are going to be higher, you will get the rest from the generators. Thus, your costs will be significantly lower.
b) An individual/company can use synchronisation in order to be self-producer of electricity (after taking in consideration any legal obligations) and outsource that energy to the main electricity network.