When it comes to installing heavy apps, it’s an advantage if you can do it offline. Download from the web, and then install Apps from microSD Card or Phone Storage or anywhere else. This feature was supported in Windows Phone 8.1, and has improved in Windows 10 Mobile.

Instead of letting Store figure out where the installation file is on the phone, all you need to do is, find the APPX file using File explorer, and tap on it. This kickstarts the installation. Here is quick video demonstration of this feature, followed by more details:

Download The Files using this Tool:

You can download apps using this tool https://wpsnitch.appspot.com. Just get the URL from the Web Store, and paste it here. Click submit, and you get more details and download link of the app.

Pre-Requisites:

Go to Settings > Update & Recover > For Developers
You have three options here. First that allows to install apps from Windows Store only, second lets you sideload apps from other sources which you true, and the last option is developer mode which allows you to install signed apps.
Depending on from where you are installing, choose an option.

Developer Options in Windows 10 Mobile

Steps to Install Apps from SD Card / Phone Storage in Windows 10 Mobile

Copy an APPX file from your Computer to Phone Storage or SD Card. You can also download it from your email or any website.

Appx file on Windows 10 Mobile

Now tap on the APPX file, and it will prompt if you would want to install the app.

Install Apps Prompt Windows 10 Mobile

Choose yes, and give it sometime. It takes around 20 seconds for the app to appear in the app list.

The Problem:

While this sounds good, Windows Store has disabled the feature which allowed one to download the app from store, and use it for offline installation. This was one of the features in Windows Phone 8.1 that was helpful for many who did not have high-speed internet on their phone. I expect Microsoft to bring that feature back in near future.

Let us know what you think about this feature in the comments section, and if you would want Microsoft to turn on this option again on the web.