I have searched about the elements inside an sdr, and I could not understant why you receive an IQ signal in your computer. However, this blog post explain very well the answer, IQ Data Explained.

The simple conclusion of this reading, at least the main point, is that when you send the singnal itself you can’t determine if is a sin or cos, the frequency, references…. but when you send the iq signal you can get a lot of information. Also some operations are imposible to perform without the iq signal, as sum two signals, multiply.