[lmilcin]

A lot of problems don't scale that easily. You may want to prolong the life of the solution by just giving it faster storage. Think in terms of giving faster storage for Oracle DB vs migrating your app to some better scaling nosql solution. SCM might seem expensive until you see the bill for the development costs.

Another use is to have local storage. You want most of the communication to be between your CPU and RAM/SCM and networking only to transmit results. In case of mongodb that would be something like very heavy aggregation that needs to look at a lot of data but transfers relatively small amount of results to the client (think counting number of objects meeting an arbitrary set of specifications). MongoDB actually requires storage for writing intermediate results of some types of operations so it is not purely about read speed.