Originally posted by Cougarific:

I found at least two traps that needed the wirecutters before I had them, now I have them and can't remember where they were.

You use them to turn off traps mostly. You can buy them from the merchant in The Crippled Burrick tavern after you beat second chapter.

There's several locations in Stonemarket where you can use the wire cutter. One in particular lowers a bridge to access a client mission later in the game.