Squid isn’t just a proxy server, its also a cache server. That means that while squid can do handle http or ftp requests for clients transparently, it can also save the response for other clients.
This is great - it can seriously speed up network access, but what if the squid server crashes or becomes overloaded? To prevent a total outage, you can setup several squid servers and load balance between them. That’s what I did using pfsense as a local network load balancer. So far its working really well, and I can take down one of the machines without any consequence. Its pretty neat actually, the pfsense bridge pings each server occasionally to test whether it is alive or not.