Got ArcGIS Desktop? Well you have a seat for your ArcGIS Online for Organisation account. Alternatively, go sign up for a free ArcGIS Developer Plan and you can make use of that account within ArcGIS for Desktop.
At 10.2, there is tighter integration between the ArcGIS for Desktop environment and the ever growing list of analysis services available on ArcGIS Online.
Sometimes you dont have the neccessary data (street networks, address locators, elevation models) or the relevant extensions (Network Analyst, 3D Analyst) to answer some geographic questions relating to your organisational data.
This is where the Ready-to-Use Services from ArcGIS Online can come in handy for you, the ArcGIS for Desktop Professional.
I put together some fairly rough videos of myself exploring some of these services. Be aware that they do consume credits, hence making use of your Developer Plan account is a good way of exploring what benefits these services can bring to your Desktop workflows.