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 about 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; Router at 10.1.1.1, with IP leases given out starting at
10.1.1.100. So, the first device to request an IP address gets 10.1.1.100,
next device gets 10.1.1.101, etc.

In this case, the addresses 10.1.1.2-10.10.1.99 are available for static
IP address assignments, like servers (e.g. servers starting at 10.1.1.10,
.11, .12, etc., printers at 10.1.1.20, .21, .22, etc.).

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.