What does the 'Secondary Use' key do?

I'm rebinding melee and skill power activation to keys closer to the WASD keys to make them more accessible. I was going to rebind Q, but by default it is set to "Secondary Use".

The only use of that key in-game that I've noticed so far is that it allows you to shop with NPCs instead of just talking to them. Are there any other uses for this key?

Secondary use button is used when you can chose between multiple interactions with an NPC;like with Marcus and Dr.Zed where you can either use "E" to simply talk to them or use "Q" to enter their shop interface.

Another example is when you finish a certain quest for Claptrap and you can press "Q" to ask him for a high-five.

As you and @dave-mcclelland mentioned, Secondary Use is used for:

• for NPCs and items that have multiple types of interactions: opening the store (like Marcus for example), giving Claptrap a high-five, turning off radios, etc.
• dropping items**

** Though right now it appears to be a bug (I assume) that even though Secondary Use can be remapped on the PC, dropping items stays with the hard-coded Q key — according to @Brysonic. I tested this in my PC copy too.