Very recently, Google added another new feature to the side panel in Gmail. Below the buttons for Calendar, Keep and Tasks, you will now see a button for Contacts. If you don't see it yet, the feature is still rolling out to domains as we speak and it will appear soon.
The Contacts button on the side panel gives you a couple of new options for accessing information and communicating with people who send you emails. One way to utilize this feature is to open an email and hover your mouse over the name of the sender as shown below.