Singularity allows instantiating an alternative operating system environment on Linux systems.

In other words:

• If you need to develop software in a Scientific Linux 7 environment on your Ubuntu 16.04 desktop and then execute it on a Scientific Linux 6 system, Singularity makes that possible.
• If you need to run software from Ubuntu 16.04 (like an up to date PDF viewer) or other software requiring recent versions of some shared libraries (like the Atom editor) on the ancient SL6, Singularity makes that possible.
• If you still have software for the discontinued SL5 around and need to run, build, develop or debug it, Singularity makes that possible.

And all this without significant overhead.