Comment by JackC

I'll add "reduce code size and complexity" to the list of benefits. A python library to calculate a simhash, or track changes on a django model, or auto generate test fixtures, will often be 90% configuration cruft for other usecases, and 10% the code your app actually cares about. Reading the library and extracting and finetuning the core logic makes you responsible for the bugs in the 10%, but no longer affected by bugs in the 90%.