Icondrop by iconscout is a plugin for Sketch and other design tools, that lets you drop SVG easily into projects. For typical UI elements such as arrows, or menu items, you’ll find yourself using icondrop for most interface design needs. There are enough free ones in there to complete most standard UI needs.

The impact on workflow is amazing, and like most Sketch plugins, they become part of the design process. However, icondrop gets used for the icons used extensively in mobile development, as
It has a simple search function with filters to allow the sourcing of the free content easily.
Personal experience
I use this all the time, it’s near perfect integration into Sketch makes life easy for rapid prototyping.
Use cases
When you need icons for apps and websites.
Cons
Minor niggle, you have to create a group object to import the icon – but it’s forgiven once you get used to it.