The web interfaces use SDRs. They can listen to an entire band at one time rather than just a single station.
You will only be able to listen to transmissions that are within range of a web SDR. They work great for HF since that propagates a long ways when the band is open. For VHF and up, you will likely need a local receiver unless there happens to be a web SDR near you that covers the band you want to listen to.
The web SDRs may have better antennas than you do and they are probably in a place with much less RFI than you. You can use them to listen to far away places.
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