what is function of primary dns suffix?

Answer 1

If you want the router to create fully specified domain names, then you may want to specify a dns suffix.

For example, if your computer name is TheBox and you added a dns suffix as thedomain.net. Then computers in your network would be able to identify your computer as TheBox.thedomain.net.

You could use this just in case you had a different network naming convention also in use.