This file is handy for status viewing tools in user mode, which can scan the text format and ignore most of it. More detailed device status (including class and vendor status) is available from device-specific files. For information about the current format of this file, see the Documentation/usb/proc_usb_info.txt file in your Linux kernel sources.

Otherwise the main use for this file from programs is to poll() it to get notifications of usb devices as they're plugged or unplugged. To see what changed, you'd need to read the file and compare "before" and "after" contents, scan the filesystem, or see its hotplug event.