USB Mass Storage Device (MSD) class is very popular among microcontroller developers. 

An MSD class device can look like a standard disk drive, one that happens to be connected to the PC over a USB cable.  MSD devices have been made popular by the handy, useful, inexpensive little USB "thumb drives" that are available everywhere these days.

Microcontroller developers like the idea of making a datalogger that can collect a lot of data (whether or not they are connected to a PC at the time), and then the device can be connected to a PC to upload its data by just a drag and drop operation in Windows Explorer.

This is seen by many developers as being easier than having to write a Windows program to upload the data from the datalogger.  Unfortunately, this doesn't necessarily turn out to be true in all cases.