When most people are first introduced to the Common App, they are told quite forcefully that this means the same application is going to be submitted to all universities, hence the name ‘common’ app. That, it turns out is not quite the case.
There are a number of possible reasons people use this. The most vehemently opposed reason is simply because people want to submit different things to different colleges for whatever reason. If anything, I find that it is quite reasonable to want to submit different essays. And even to have different activities. The simple fact is that you might want to emphasize and highlight different things in your application to different universities. Certainly, it doesn’t really help your workload given the mountain of things the Common App already requires, but it is a legitimate choice.
On the less objectionable end of the scale is perhaps the situation faced by people who submit applications early. Since your early application is then locked, it is equally logical that you may have new thoughts and ideas come the deadline for your other colleges.
Regardless of the reason, the possibility to do so is readily available. If anything, it is provided by Common App themselves. You simply create ‘alternate versions’ of your application, select different colleges for the respective versions and submit as you usually would. Of course, one must be careful that the right version is submitted to the right college and to keep track of all the various versions. But that’s no reason not to do it if you have the need and have the ability to manage yourself.