ok, just to give you an example.
I have an SIP phone, I also have a Wireless printer/scanner.
Both of these have a Web interface, so I need to know that they are always on the same IP.
However it is difficult to configure them to use a static IP with all the settings that come with it, so in this case I use a static lease.
I also have a Linux server that I can connect to in various ways, and although it is easy to configure the IP settings on it, why would I when I have a DHCP server on the network that can provide all the settings?