domain name system – static DNS entry, DHCP reservation, advantages of static IP differences

I've been looking a little bit now.
And I wondered what are the benefits and risks of doing such things.
I know for example that our firewall can handle the DNS resolution as well as the IP addresses themselves.

I know for example that if a user has a certain PC, he sometimes has a static IP or DHCP reservation and / or a static DNS entry.

But how do you decide this or what you should not do at all.

I am a little "new" and I confused recently.
Maybe someone can explain or remove the confusion.

Thank you in advance for all the useful contributions