Google Enterprise - replaces Web server, mail server, DNS server, chat server, and NAS devices.
Solidworks SaaS - replaces simulation resources with cloud resources.
Slack SaaS - chat servers, administration of same.
There is an IT concept, SUDs, which is short for "Servers Under Desks." That is a situation where an employee sets up a server do do something and it "lives" under their desk because they don't have an official place to put it. And then it becomes mission critical without anyone knowing and there is a big fire drill when something disrupts the flow (employee leaves, server crashes/fails, scaling requirement isn't met, Etc)
That's usually not the case. SaaS usually does not replace in house machines, and it usually does not deal with high performance applications.