What is Sideload feature?
App sideloading, from a SharePoint context, is the ability to install a SharePoint app directly into a site to explicitly bypass the regular governance controls of SharePoint.
Why is it Useful?
The developer site is designed specifically for app developers to install and test apps before they are released to the store. There are certain site collection templates, such as Team Site and Project, which are not compatible to deployed app directly. This means that developers cannot test apps designed for use with those templates with the developer site alone. The sideloading feature enables development against site collections that use a different template without affecting the UI. By enabling sideloading, an admin opens the door for anyone with app install permissions to install apps that are not sourced from the app catalog or app store.