Join messages can have several uses:
inform members when someone uses their invitation
log invitations to find out who invited which member (useful during a raid, for example, to find out who invited spammers)
be used for statistical purposes, to find out, for example, whether members join via a site such as Disboard.org or by inviting each other
Once you are connected to the dashboard, click on the Manage button, under the name of your Discord.
Enter a message (it will be sent when a member joins the server) and select a channel (the one in which the messages will be sent), then click Enable the messages.