Windows

Point value changed by question poster.

The only benefit is the following: if you use reservations for ALL your computers, and your IP's subnet range ready for leas by your DHCP is restricted to those 30 IPs, no new computer will be able to connect to your network unless you give to it the rights of doing that!

One security benefit that I can think of is that you may want to keep a grouping of workstations
with contiguous IP addresses for firewall rules.
For example, no outbound port 80 for 10.1.1.20-25
You could also could prevent the DHCP server from giving out IP addresses by keeping your reservations in an exclusion zone and configuring DHCP not to distribute IP addresses past the exclusion zone. An unauhorized person trying to connect to your network would have to know the private IP range/subnet you are using to get onto your network. Although this doesn't give your network a level of High security, it makes it more difficult.