Several new USB devices have their proprietary Windows drivers onboard, especially WAN dongles. When plugged in for the first time, they act like a flash storage and start installing the driver from there. If the driver is already installed, the storage device vanishes and a new device, such as an USB modem, shows up. This is called the "ZeroCD" feature.
On Debian, this is not needed, since the driver is included as a Linux kernel module, such as "usbserial". However, the device still shows up as "usb-storage" by default. usb-modeswitch solves that issue by sending the command which actually performs the switching of the device from "usb-storage" to "usbserial".
This package contains the binaries and the brother scripts.