Whenever the Exchanges are upgrading their trading systems, hardware, software or want to conduct a Disaster recovery site test they conduct a Special trading session/mock session to ensure that all the necessary systems are in place.
Disaster Recovery site is a replica of the main site which is in place to ensure that in case there is any disaster at the original business site there are sufficient systems to fallback to, to ensure business continuity. Its like having a Plan B at the time of a crisis. The mock session also allows trading members to check whether their approved algo strategies/systems are working fine. SEBI has made it mandatory for all stock brokers to compulsarily participate and test all their approved algo strategies in all mock sessions.
The difference between a mock and special trading session is that in a mock session, the trades that take place are dummy unlike a special session where the trades are real.
It is similar to overhauling your car to check if all is well and taking it out for a test drive before the race day.
There is no benefit as such for the retail client except that he can participate in these sessions.