Should I keep it, or is it something I can delete?

You can delete it. It's not needed to run your mod. I think it's just supposed to be a temp file while generating your bsa.

I think the archiver may have use of some certain data from it it for decompression, like a structure.

I use BSAOpt anyways for getting things done.

Ok cool, thanks

It's used by archive.exe if you run that to unpack the bsa file. Otherwise serves no purpose.

So, all the BSL files that came with some of the mods I downloaded (like the Unofficial Skyrim and DLC Patches and Run For Your Lives) are superfluous, useless and can be deleted? If so, I wonder why they were included then?

Just in case you would need to unpack the bsas with Archive.exe, probably.

So if I understand this correctly, using another BSA unpacker such as BSAOpt or old FOMM does not require these BSL files, but using Archive.exe does? A bit odd, but okay then.

That's Bethesda's system for you. Archive.exe needs them. Other BSA utilities probably don't. If you don't ever intend to unpack the contents of those files then you don't need the .bsl files.