The sampling rate establishes how wide a frequency span you can see at a time, Nyqist established that the sampling rate must be at least twice the frequency of interest. For 48Khz sampling the computer can see a 24KHz signal, but since we are sampling two signals we actually can see + and - 24KHz for a total of 48KHz of signals. Taking that further if we have a high quality card that can sample at 192KHz then we can see 192KHz segment of the band at one time, not bad for a $11 radio.

The beauty of this is that you engage a second set of sense when using your radio, your sight, you can look a a fair size portion of a band at a time and see where the signals are, and even what kind of signals the are, it a wonderful experience that can get you hooked in no time.

A image of my computer desktop running PowerSDR software, with a SoftRock40 looking at a portion of the CW band

Note the CW signals rising above the noise floor of the receiver.