1. What is the purpose of a seat occupancy sensor emulator (seat mat emulator)?

Installed instead of a passenger seat mat, it sends to an airbag control unit the same signal as that sent by a mat while a passenger is sitting - a car computer always sees the seat as occupied. It makes the SRS light off and airbag always work as if the passenger was sitting. In case of a collision airbag will be always deployed.

2.  Do I need a seat belt emulator as well?

As a result of installing the seat occupancy sensor emulator, the car computer sees the passenger seat as occupied. In car models with a seat belt warning system, the seat belt emulator will switch off the warning light or/and sound that the passenger's seat belt is not fastened. The seat belt emulator sends to a car computer information that the seat belt is fastened - it switches off the seat belt warning. 

If you don't have a seat belt warning system, you don't need the seat belt emulator.