Key things it appears to do:

•     Browse other After Effects project files directly from inside AE
•     Import only the compositions you need
•     Avoid importing entire project structures unnecessarily
•     Speed up template/library workflows for motion designers
•     Help reuse animation setups across projects

It’s especially useful if you:

•     maintain a reusable animation library,
•     work with many client projects,
•     frequently copy precomps between projects,
•     or build modular motion graphics systems.

The current listed price on aescripts is around $29 USD, with periodic discounts.  

Related tools/workflows people often compare with it:

•     LayerStash for saving reusable layers/setups
•     True Comp Duplicator for duplicating comp hierarchies
•     compCode for reusable project structures