Google Now on Tap was introduced alongside Android 6.0 Marshmallow, which used context on your screen to help you find information. Google Assistant replaced Now on Tap (for compatible devices), and carried over most of its functionality. Now Assistant is making it slightly more obvious that it can search your screen and take screenshots.
Left: Old design; Center, Right: New design
Before, screen search results would appear below the main Assistant prompt after a few seconds, and the screenshot button was found below the screen search results. Buttons for screen search and taking a screenshot are now immediately visible when Assistant is opened.
This is a minor improvement, but definitely a welcome one. The change appears to be part of a server-side update, so if you don't have it, you will soon.