Interlacing is how television was designed to work in the old days because of a lack of technology to draw entire pictures on a screen. If you want to see a hilarious, but extremely accurate explanation of interlacing,
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Kind of a long watch, but it's got a great explanation of interlacing as told in the 1950's, which ironically is different than you'll hear told today.
Virtually ALL television broadcasts are interlaced, and even progressive scan devices output interlaced information at some point if connected to a television that isn't progressive scan. By comparison, there might be 1 progressive scan display in the home viewing world for every half million televisions. Progressive scan for television is very much in it's infancy, but it's rockin' and rollin' along very, very fast.