You ask your friend to send you the contact details of the best photographers she knows for an event you're planning. Or your parents ask you for the number of a couple of good doctors they might call for a consult. These are just two of many circumstances where you might need to share the contact details of several people at the same time.
Previously, WhatsApp only allowed you to attach one contact at a time in a chat, forcing you to repeat your taps several times just to be able to send several persons' phone numbers and/or email addresses to your friends or family. But at some point in the latest beta versions (we're inclined to think 2.17.122 or 2.17.123), the option to select multiple contacts was enabled.
As you can see, it's as easy as selecting multiple photos to share or as picking the people to start a group with or send a broadcast to. You tap and hold the first contact and you can select several then share them all in the chat.
The option is available in WhatsApp 2.17.123, which you can find on APK Mirror or by joining the official Play Store beta, but as I said, it might have been added a few versions earlier.