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.
That guy gives some _amazingly_ interesting talks - I got into SDR pretty much purely based on a presentation of his at DorkBot in Sydney a long time back...
I can't find the link here right now, but there's some more recent work that's doing that without needing the two clock synced receivers too...
It's a course that relies heavily on MATLAB though.
It's always nice to see RF and Signal Processing stuff on HN.
If you're seriously interested, download Octave (Matlab replacement) and write a program from scratch to simulate a BPSK system and generate a "waterfall" (bit error vs. noise) curve. Don't stop until you can get your curve to perfectly match the one in a text book . By doing this, you will learn about modulation, demodulation and and "Additive White Gaussian Noise" (AWGN) channel models. If you can get the waterfall curve to match exactly, you will also be covering concepts such as "energy per bit". A BPSK should be quite doable for an amateur.
Once you have the BPSK simulation, try extending your simulation in different directions and matching the textbook curves. The following challeges are roughly in order of increasing difficulty
1) Add a block code, such as Hamming or BCH.
2) Replace the simple AWGN channel model with a Rayleigh or Ricean model
3) Add a convolutional code (with Viterbi decoder)
4) Implement Orthogonal Frequency Division Multiplexing (OFDM)
5) Implement an LMS receiver
6) Implement a Low-Density Parity Check code (use the one from the WiFi standard)
7) Implement a complete 802.11a simulation
8) Implement a complete DVB-T simulation
9) Implement a MIMO system, assuming perfect channel knowledge
10) Implement a MIMO system with channel estimation
Get to number 10, and you will know more about radio signal processing than most engineers.
 eg. "Lin and Costello" Error Control Coding or "Proakis" DSP
there is the canonical Fundamentals of Wireless Communication (https://people.eecs.berkeley.edu/~dtse/book.html) which is pretty good. also, iirc, there is a book by oppenheim as well (signals and systems, i think)
edit-00k : yes, oppneheim book is called 'signals and systems' and it is available on ocw as well (https://ocw.mit.edu/resources/res-6-007-signals-and-systems-...)
Are there any other comparable options?
RTL Dongles -> ADALM -> HackRF-1 -> LimeSDR -> Ettus SRP
The RTL's don't transmit but all of the other ones can.