There are literally lots of reasons. Some of the most common would be if
you have a web server, you may have 5 websites and want to use a different
IP number for each of them (instead of host headers for example).
Another reason would be if you want a few IP numbers assigned to each for
dns reasons.
Yet another would be if you want to temporarily assign services used for one
server to "temporary server", you could assign the additional IP number,
then dns would point to the temporary server and greatly minimize the work.
All these reasons are great for servers. As for a workstation, there are
not a lot of reasons that I could think of except for one. If you have a
laptop and are "bouncing" between locations, you could assign multiple IP
numbers based on the scheme of the particular locations, such as 10.1.1.100,
192.168.1.100, etc.
Randy