When assigning static IP addresses, you want to assign them outside the DHCP lease pool, and that generally means excluding a portion of the range from the DHCP pool. If you assign a local static IP that is in the DHCP lease pool, the DHCP service will not know it, and may potentially assign the same IP address to a different client device, causing an IP address conflict and communication problems for both devices.
For instance, a router at 10.1.1.1 will give out IP leases starting at 10.1.1.100. That means the first device to request an IP address gets 10.1.1.100, the next device gets 10.1.1.101, and so on.
In this case, the addresses 10.1.1.2 - 10.10.1.99 are available for static IP address assignments. For example, you can assign servers starting at 10.1.1.10 then .11, .12, and so on. You can also start printers at 10.1.1.20 then go to .21, .22, and so on.
If you manually configure a device (e.g. printer, server, access point, etc.) to 10.1.1.110, then a network with than 10 other dynamic devices on your network, one of them will boot up, ask for an IP from DHCP, then DHCP may lease out 10.1.1.110 because it doesn't know it has already been taken..
Some DHCP services have an "exclusion range" into which you enter IP addresses they are never to use, because you've programmed some devices locally with those IPs. (i.e. servers are commonly locally assigned IPs not through reservations). Usually, devices that need to be accessed, either locally or remotely, require static IP addresses.
One can also use the DHCP service for "reservations" where a client device is always assigned the same static IP address. This is typically used for printers or other devices that may not support being programmed with a static IP address, but a static IP address for the device is desired for monitoring or access.