Favicon is also known as a shortcut icon, website icon, tab icon, URL icon, or bookmark icon, or a file containing one or more small icons, associated with a particular website or web page. Nowadays, they are very important.
They have been prominent in Browsers because they are very helpful in distinguishing one website from the other one in your tabs at a glance.
Inspiring from this idea, Gmail has also taken the step to make it more effective and efficient, for this purpose they also have introduced the same thing in your Gmail’s inbox.
Favicon has been added, along with the website domain of a sender’s email address, to Gmail Sender Icon as a small image in your inbox.
This will help users to go through the important emails quickly and then to the unwanted emails. As it is a small visual trick but it is more effective and efficient than the older one.