Why: Assign a custom Home page for different apps, so that users that live in different apps don’t use the same layout. This additional layer of assignment makes designing a Home page simpler, so you can specify different customizations for different app users.
For example, the Home page for your org’s inside sales app can have a different layout and different components than the Home page for your field sales app, each focused on connecting reps in those areas with the tools and info they need.
Setting an app default Home page doesn’t impact existing profile assignments. Existing profile assignments are nested beneath the layer of app assignments. As a result, users with different profiles can see different Home pages within the same app.
How: To assign a Home page as an app default, from Setup, enter Home in the Quick Find box, and then select Home. You can also assign specific Home pages from the Lightning App Builder.