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Excuse my ignorance, I have only visited one salt mine, and it was a working one in the US.
There was only one entrance, a tiny elevator that held about 20 people standing packed in. 
I'm assuming there was a different way they removed the salt, and maybe in a disused mine, that's a good way to get large, heavy objects in and out. But our guide told us equipment had to be brought in through the small elevator, piece by piece, and would never be taken out.
There was an area used for storage of precious objects, kind of used as a museum here, but none of them were large objects.
This mine was quite deep, hundreds of feet if I recall correctly, not a place to get in/out of quickly; and again, only about 20 people at a time.
And not many toilets. All waste, trash, food and water had to be hauled up and down.
For anyone who understands better than I do, besides storage of small items and maybe a couple hundred people at a time (logistics of bringing in food and water for large numbers of humans), what is the strategic benefit?
Is this salt mine very different in structure, depth, operation?
I don’t envy the fight that’s about to come.
Tunnel warfare is no joke. 
You’re deep underground, less clean air, fighting in darkness (if power to the lights is turned off) - worse if you lose your sense of direction, plus all the deafening sounds from gunfire and explosions, amplified lethality from grenades, little to no cover/restricted movement.
These salt mines are supposedly massive with several entrances/routes. You could be outflanked and not see it coming. >

Why do they have to fight in the tunnels?  Russia can just work to encircle the mine.  And the tunnels probably aren't accessible to resupply easy. It makes no sense to post up in the tunnels here. This isn't Vietnam. >

I heard that Russia may want to use the mines as an ammo depot. Not sure how accurate that is though. >

The only safe place from HIMARS? >

Ukraine knows all the entries to those tunnels, they have guided ammunition to blow them up, I don't understand why people think that Russia would be safe by trapping themselves into that kind of hell. >

If it's the choice between attacking one location that goes boom and attacking several locations that only collapse, then I'd say that the latter is much more difficult to achieve. Especially if the collapse isn't big enough as it'd then be possible to dig up again in a timely manner.
The big question is how safe are Russias ammo depots in Ukrainian territory? If Russia is completely unable to hide them then choosing a location like the mine would actually be a good idea since Ukraine would attack them anyways and a mine is much easier to defend and would give more room for mistakes.
In addition to this there are so many things that play a role, like how easy is it to collapse those tunnel? Would Russia be able to dig them up again? How many entrances are there? How safe are other Russian ammo depots? Is Russia able to deploy enough anti-air systems in the area? How extensive is the mine? How many entrances have good road access? Has Ukraine already booby trapped the mine?
Honestly, there are so many points that come into play here and all of this is so far out of my area of expertise that I can't say whether it's just bullshit that I heard about or whether it's actually reliable.