It's for verifying the correctness of subtitle files. The whole subtitling world is very, very messy. There are dozens of file formats, many of which have many variants and versions. You might have a file that works in one place, but when you upload it somewhere else, it doesn't work.

Trying to figure out why means reading dozens of pages on dozens of sites trying to understand that 'Oh SRT formatting doesn't support that particular styling format'. It's a mess. Also dumb stuff like a subtitle that has zero length or a subtitle that has a end time before its start time. Trying to understand whats broken in a subtitle file is a PITA.

This is the first tool I've seen trying to make it easy to figure out if a subtitle file is broken.