'Cash' generally allows you to buy in-game items, which includes resource boosts, speed-ups, combat bonuses, crates, etc.
Buildings require 'General Orders' to be leveled-up to level 10, and of course, 'General Orders' requires 'Cash' to purchase.
You recieve bonus crates for various amounts of 'Cash' that you purchase (NOTE: FIRST PURCHASE ONLY):
- Beginner's Crate - 200 'Cash'.
- Lieutenant's Crate - 300 'Cash'
- Major's Crate - 525 'Cash'
- General's Crate - 1100'Cash'