Have you noticed when someone joins a Google Doc their image appears in the upper right hand corner. If the person is not shared on the document they appear as an anonymous animal. In other words if the document is not private, then those who click on the link to view the document are assigned a random animal to represent them.
The obvious purpose of icons is to let you know that other people are viewing your document. If the document is expressly shared with someone, you can know who is viewing the doc.
If you are collaborating on a multi-page document or on a Google Slides presentation you may want to view the work that others are contributing. If you are working on the document at the same time you can JUMP to the point in the document or slide in the presentation where another person is working.
If Mary is typing on page 10 and you are working on page 3, simply click on the Mary’s icon to JUMP to page 10. Clicking on a persons icon will jump you to the spot they are working on. If the person is anonymous, and the doc is set so anyone can edit, you can jump to the slide where the chupacabra is working on the presentation.