It is a popular design convention, there are even generators to convert regular screenshots into this angled and depth-of-field style. There are 2 important properties this convention has:

First, it makes it obvious that the UI being presented is not part of the website, but a screenshot, so people don't trying to mess with it like it's an interactive widget.

Second, it showcases a general feature without inviting scrutinising the details of the UI, which a general product page isn't the best place to do so anyways.

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(Personally I do love a good software Screenshots page, but that can be a separate thing IMO)