It is a full standalone engine management system, meaning it can be tuned to whatever configuration you want.
Its purpose is to increase power, even from stock engine you can get more power, though not more than ~10% on a healthy engine.
It is MUCH easier to tune than any method with original DME and it has integrated wbo2 controller, just connect a sensor. Does original ECU has dual WBo2 and dual EGT controller on board? No.
Why would you need different maps? Only reason I could think of is if you use E85 for example occasionally and you need to switch maps. VEMS can store two maps and supports flexfuel sensor too.
For different engine mods you can tune thousands of maps in your computer and upload them in a minute.
It can be tuned using its autotune:
https://www.youtube.com/watch?v=U7fyfhY_FhI
If your car has original cats it will pass the emission test with flying colours given that you have tuned the fuel to match your engine configuration. It has closed loop WBo2 control which can be configured any way you want or disabled.
You can download VEMSTune software from here and browse through features etc:
Vems.hu - VemsTune main site