Linking your Steam and Discord accounts gives you extra perks on the Rust server. It does not give anything special on the DayZ server. To see the perks for linking your accounts, go to the #account-linking channel in Discord.
The process of linking your accounts is very simple.
1. Go to https://link.rustyoldgamers.net/
2. Click on Get Started and sign into Steam.
3. Click on the Discord Member button to link Discord and Steam. If you are a member of the Steam group, click on Steam Group member.
If you boost the Discord server after linking your account, just log back into https://link.rustyoldgamers.net/ and click on Check again.
If you have any problems with these steps, please submit a support ticket in Discord.