dnsadmin is a cPanel developed script that acts as an intermediary API between WHM and BIND to increase performance of DNS record updates.
From cPanel's release notes:
"dnsadmin, the system that cPanel & WHM uses to perform DNS updates can now be run as a service (daemon). This eliminates the need for it to startup and shutdown each time a DNS change is made. During testing, it was able to process 22x as many DNS update requests in the same time. This should increase performance for users running DNSONLY."
It's not required for DNS to operate, however on large servers hosting 1000's of zones, it's performance benefit can be seen.
Occasionally you may see a notification email such as:
This is not a fault. cPanel will regularly unload services from memory if they have been inactive for 10 minutes. This behaviour can be adjusted in "Home >> Server Configuration >> Tweak Settings >> Software"