What can it do that other software can't?
A lot. List would be too long to post it here. Besides, there were various versions of software / hardware combinations, on NuBus and PCI and also various options the software was keyed (!) for.
Is it convenient?
No. Steep learning curve.
What is it mainly used for?
For many years it was de-facto industry standard for CD mastering / classical editing / film and radio work.
Is it worth me getting for anything?
Unless you can find working system which includes all hardware and valid key file, stay away. Without the valid key file the system won't run. Key file was produced individually for each system and included board(s) serial numbers and options (NoNoise, SRC etc.) that were purchased. No support from Sonic Studio anymore and it has never been k'ed, if that's what you've been thinking about.
And, even if you manage to find working system, be prepared to deal with proprietary file system, finding the correct narrow SCSI drives, software bugs and so on. Sonic software, even up to this day never really left the beta stage.