In this project, there are two drag and drop interactions. In the first one, you can drag and drop people behind yellow lines, so that they maintain safe distance amongst each other. In the second interaction, you can drag and drop people, and make them sit far apart.

What is that example intending to do? When should the user use such a theme?

In the first drag and drop interaction, you will see the use of many to many interaction, where any person can be placed behind any yellow line and at a time only one person can be placed behind each yellow line. If another person is dropped there, the previously dropped  person is sent back to his original position.

The second interaction is a simple one-to-one interaction, where you have to drag and drop the images to their silhouettes. Only the correct match is accepted and the others are sent back to their original position.