Let’s say you run a front end system like a reverse proxy, load balancer or http accelerator (e.g. big-ip, squid, pound, varnish, apache, nginx, etc), which pass dynamic requests to back end application servers, then you’re likely to have to deal with your applications seeing client requests as coming from the IP(s) of your front end boxes.
This presents a slew of quality of life reducing issues, unless of course your back end system is Apache, in which case you can use mod_rpaf to make your life pleasant again.