It allows to listen to FM radio, decode most 433MHz devices (weather stations), car keys signals, and even NOAA weather satellites  with a DIY antenna .
I can read about 40 smart meters from inside my home using the antenna that comes with it.
Said from experience of having built something more complicated and having it perform worse than two pieces of copper tubing stuck in a piece of scrap plastic.
EDIT: Added links I found helpful, for the curious:
Proximity cards used for door access usually have the 125kHz carrier compatible with the EM-Marin EM4100. No cell phones I know of have an antenna for this frequency range; therefore, no phone can read, clone or emulate an EM-Marin proximity card.
Since the HID Proxcard II is a "value priced 125 kHz proximity card", you cannot use a phone to read it.
This would allow you to say, hold one device to the reader and one against someone's pocket to open a door.
Or to share an NFC transit pass between multiple people over the internet.
No one has a proven system for doing this in today's smartcards to my knowledge. Though there is some research which promises this may be possible in the future.
Even those proximity car locks do a horrible job of distance bounding - many of them do it off RF level which means an attacker merely needs an amplifier to steal your car - and that offers an almost optimal situation for the application. So I think we'll probably see it there before smart cards. Maybe the timers necessary make this cost prohibitive though. I'm not in a position to say one way or the other.
Better (and more expensive) RFID tags may have an encrypted communication protocol.