It seems that not everyone can agree on what to call parts in a 3D CAD environment. For example SolidWorks calls them "parts" whereas AutoCAD calls them a "body" or component. I shall use the word "part" to mean individual parts, components, or bodies that can be added to an assembly. I think we can all agree with that.
SolidWorks has an extensive Design Library that stores common parts and parts features. For example the SolidWorks Premium 2015 x64 Edition (SP 3.0) comes with the following features which can easily be imported to parts: fluid power ports, hole patterns, keyways, sheetmetal, o-ring grooves, retaining ring grooves, and slots. They also have forming tools, motion objects, common parts especially hardware, routing (electrical, HVAC, tubing, and piping), mold bases, and what they call smart components. Libraries such as these save a lot of time and cuts down on monotonous repetitive tasks when designing parts. But I think the its important function is that of user defined custom features and parts that a user can easily add to their design library without the need to do any coding.
Of course having the ambition of trying to implement too much would be a very costly and futile endeavor for parts or features that most users wouldn't even need. Yes AutoDesk's Fusion 360 already has many fine tools for creating features or parts but it still feels incomplete. Why not make an addition drop down menu icon for a design library that is easily extensible by users without having to do any coding? Of course this library can also be extended by third party partners. The main idea is to create an infrastructure for easily managing custom parts and features.
See image below for a print screen of Design Library by SolidWorks. The fourth icon from the left with the plus sign on it is for adding custom parts or features to the library.