It is very useful to compare images from different dates or even between different indexes. To quickly access this option, click on the layer selector and choose the first image you want to compare.
Then, in the lower right corner select the option A | B and you will enter the comparison mode.
The next step is very easy, choose the second image from the date selector on the right.
To return to the simple viewer, click on the same lower right corner where it says A.