Coffee has lots of micronutrients in it, so it's used as a fertilizer. Coffee grounds offer the same advantage, plus they provide acidity, and are a good fibrous organic component that both retain moisture and promote good drainage. Or at least, that's what I've inferred.
Most Starbucks locations will give you free used grounds for use in your garden if you ask nicely. I make my coffee fertilizer from it by leaving it to boil in a large stockpot with RO water for several hours. I think that real coffee might be better than instant, but that's just a hunch - I've got no evidence to back it up.
If you're just getting started with Neps, though, skip the coffee altogether; it's not at all necessary. Learn to grow them the conventional way before you get fancy.
~Joe