DHCP versus static: how do you choose?

In my networking textbook, it specifies that sometimes, server administrators will pick different schemes related to DHCP and static network configurations. One of the set ups is to use the lower end of the spectrum as static and the upper end of the spectrum as DHCP. Mixing them together. Why bother making them at all? Isn’t it better to have dynamic configured hosts? Minus servers, of course, because you want those the always have a static IP address I’ve heard