What are the coins Ogres drop used for

• A combo is when an orc dies from 2 or more methods at the same time. For example, a tar trap slowing them down and a clockwork mace gibbing them will count as a combo.
• A kill streak is when you kill many orcs within a small time period. If you continue killing orcs within a second (or so) of one another, your kill streak increments. Once you've gone a second (or so) without killing an orc, you are given a bonus based on the number of orcs you killed in that streak.

These are just spare slots. The items doesn't exist yet, and are most likely to be included in future updates or DLC.