Technology

How does the web interface collect the transmissions?

The person or organization hosting the website has an antenna somewhere attached to a Software Defined Radio, or SDR. I honestly don't know how these work at the silicon level, but radio antenna feed line goes in one side, some JFM happens, and USB and/or PCIe computer data comes out the other end. Instead of tuning into such and such frequency with such and such modulation, it sends the raw RF data to the computer to let it process it digitally, with algorithms and GFLOPS and RAM and shit.

Which means, you get to tell it "process the data as if you're a single-sideband radio listening on 14.070MHz Upper Sideband" and you can listen into amateur radio slow scan television. It's basically like you get to remote control someone else's radio receiver.

Are all the transmissions made digitally accessible with the interface?

No. See the above "A person has an antenna somewhere." You can hear what that antenna hears. This will be limited to line of sight for VHF and up, and even HF will be limited by propagation conditions and the nature of the antenna. The hardware they've hooked to their computer may also have its own limitations. Also, their antenna is imperfect because there is no such thing. This is the world's shittiest Wi-Fi antenna (only partially because it fell over).

Why (other than cost) would I want to use a web interface rather than a traditional receiver?

Not all radio transmissions can be heard from everywhere. I can't hear anything above 12 meters out of eastern Europe from here, not in the worst solar cycle since humans learned the sun has cycles. I can hear it loud and clear from some Frenchman who put his SDR online.