Views can include website data from before the view was created.
A reporting view is the level in an Analytics account where you can access reports and analysis tools. Analytics automatically creates one unfiltered view for every property in your account, but you can set up multiple views on a single property. Any data you send to an Analytics property automatically appears in all views associated with that property.
Read more here: https://support.google.com/analytics/answer/2649553
Access, customize, and analyze your data in a view.
A reporting view is the level in an Analytics account where you can access reports and analysis tools.
Analytics automatically creates one unfiltered view for every property in your account, but you can set up multiple views on a single property. Any data you send to an Analytics property automatically appears in all views associated with that property. For example, if you collect data from two websites and send it to one property, then data from both websites appears in all reporting views on that property. This is also true for web and app data. If you collect data from a mobile app and from a website, and both your SDK and your tracking code send data to one property in your Analytics account, all data (both web and app hits) appear in all reporting views associated with that property.
You can use filters to customize views and see only a subset of data in your reports. You might, for example, have one view to filter out all internal traffic, and another to include only activity from a specific website directory or subdomain. If you send web and app data to the same property, you might want want to use filters to exclude web or app data from specific views. Other Analytics reporting tools, like Goals, Segments, and Alerts are all applied to individual views. You can also restrict user access to specific views so you can control who sees what data.
We recommend that you don’t delete or add filters to your original view. When you delete a view, that particular historical perspective of the data is gone. When you add filters to a view, the data you exclude is unavailable. To preserve all your original data and also control the specific perspectives of that data, create a copy of your original view or set up additional views and customize each one to meet your reporting goals.
Difference between web and app views
When you create a view, you can choose between an app view and a web view. These two view types give you a slightly different analysis experience, but are otherwise the same. For example, app views give you some reports that aren’t available in web views, like Crashes and Exceptions and the Google Play reports and web views give you Site Content reports.
Both types of views can display any data you send to your property, regardless of how you collect those hits. For example, you can see web data in your app views and app data in your web views.
If you’re only collecting and sending one type of data to Google, we recommend using views that will give you the best analysis experience. For example, if you only collect data from mobile apps, select app when you create a view and if you only collect data from websites, select web.