This “double pane” view allows you to “browse notes keeping on the left side of your screen, while editing content on the right”. If left is set to multi-column view, you’ll get two columns like on phones, but the single column should be more efficient.
On the other half, you get your note (or list, picture, etc.). Previously, Google Keep would open content in the center of the screen while the column(s) remain visible in the background. It was a fine approach, but could be somewhat distracting. This new design allows you to truly multitask.
In a help article, Google says that the “dual pane is available if your device’s screen is > 840 dp and you are using the Google Keep app in landscape mode.” Android 12L’s large screen capabilities (e.g. dual panels) start at over 600 dp.
It’s not clear if this two-pane view is automatic, or if there will be a setting to stick with the current approach.