They made a
resurgence with µ4/3, but they've been around for much longer. Zeiss's four-element Tessar design has been around since 1902. Pentax has until now, been the only dSLR brand to keep pancakes in Nikon, Olympus, and Zeiss all made 40-ish pancake lenses in their day. One of the more popular lenses to adapt onto Canon EOS bodies for a while was the Contax/Yashica-mount Zeiss Tessar T* 45mm f/2.8:
And until this announcement, the only native-mount option you currently had for Canikon cameras was the manual-focus
Cosina-Voigtlander Ultron 40mm f/2 SL II. I have a feeling their sales may dip, though, on the Canon-mount version, now that there's an AF alternative.
And yes, street shooting or travel would be the main reasons you'd want one. The profile makes you less conspicuous/threatening, and the overall bulk of the body/lens combo is greatly reduced. On m4/3, where the main attraction is the small size & weight, pancake lenses have become very much in demand, because it can make some of the body/lens combinations pocketable (or at least coat-pocketable).