Advantages of Apps for SharePoint over classic solutions
The most important guidance we can give you is to develop an app for SharePoint rather than a classic solution whenever you can. Apps for SharePoint have the following advantages over classic solutions:
They provide users with the easiest discovery, purchase, and installation process.
They provide administrators with the safest SharePoint extensions.
They provide you with the simplest marketing and sales system based on a Microsoft-provided online app store.
They maximize your flexibility in developing future upgrades.
They maximize your ability to leverage your existing non-SharePoint programming skills.
They integrate cloud-based resources in smoother and more flexible ways.
They enable your extension to have permissions that are distinct from the permissions of the user who is running the app.
They can extend SharePoint Online websites as well as on-premises SharePoint websites.