Side Load Applications in Windows 10 usually used when you want to install Windows 10 App by offline. Although on default, Windows 10 blocks access to install applications outside of the Microsoft Store.
How to Side Load Applications in Windows 10
- Press Windows key + I > Update & Security.
- Click For developers > Sideload apps.
- Click Yes on notification.
- Restart your PC.
Note: this options allow you to install Windows 10 apps offline, only Developer mode gives more options for app developers.