A ring of spell storing has 1d6-1 spells. If I find the ring with 0 spells, as I actually did, does the ring still have a use?
Yes, you can cast spells into it
Either you or someone you know or can hire who has spellcasting can put spells into the ring (DMG, 192):
Any creature can cast a spell of 1st through 5th level into the ring by touching the ring as the spell is cast.
The Ring of Spell Storing states:
While wearing this ring, you can cast any spell stored in it. The spell uses the slot level, spell save DC, spell Attack bonus, and Spellcasting Ability of the original caster, but is otherwise treated as if you cast the spell. The spell cast from the ring is no longer stored in it, freeing up space a spell by way of expending any leftover spell slots, of course. This ring stores Spells cast into it, holding them until the attuned wearer uses them.
You need to specify which spell slots are being taken from you and placed into the Ring in order for those spell slots to be utilized when using the ring as an action.
It's the end of the day and you found that you used too many cantrips (no such thing if you're still alive) and have a few spell slots kicking around. Why waste them? Save them and carry them forward via The Ring of Storing.
Just clarifying that filling the ring would in fact still cost the caster a spell slot. The ring effectively STEALS one of your spell slots so that you can use it at another given time.
This is the opposite of an Artificier at LvL 11, for instance, who can store spells into a weapon or casting focus after a Long Rest. Not affecting your daily spell slots.