If you know the Windows program ‘ArtMoney’ you already have an idea what a memory scanner/editor like Scanmem could be used for.
For other people, basically this kind of tools allow you to search for variables in memory of a running process, generally using one initial value and then a second. When you have the precise offset, you can set the value as you wish (and then have a toon of money, lives or ammo).
This kind of tool could also be used to analyze an hostile process on a compromised machine but cheating in games is way more cool ^^