Some devices need predictable IP addresses. Printers, servers, and access points work better when their IP doesn't change.
A DHCP reservation binds a MAC address to a specific IP. When that device requests an address, it always gets the same one.
Reservations give you the benefits of static IPs (predictability) with the benefits of DHCP (centralized management, automatic gateway and DNS configuration). You manage everything from one place instead of configuring each device individually.