RCM allows you to setup a sales funnel that marketo can track
https://community.marketo.com/MarketoTutorial?id=kA250000000KzWoCAK
This is what Marketo sells on - the vision of a sales funnel report with conversion data. The RCM itself is a visual model with transition trigger rules between stages. Usually I recommend setting this up with basic triggers and building the lifecycle system in a Program so you can control it better and get better data. You'll need the Success Path Analyzer too.
RCM only becomes truly powerful when you have RCE and can do more data analysis on the stages.