More technical answer.
Ok so, when your viewing a web page in a tab, your interacting with that page. When you switch to a new tab your no longer interacting with that page. This has some serious side effects with Cross site scripting protections and with javascript. A lot of javascript calls (in fact almost all of them) require the user to actually be looking at the view port (think of it like the tab) in order to run. This is a security thing, so that you don't leave a tab open while it does something like read every file on your hard drive (not that it can directly do that anyway), or more recently run that little SSL cracking javascript while your watching YouTube on another tab.
The side effect of this is that the page is "stale" when you go to view it again. Think of it like your bank. Your on your bank page, then you tab over and watch 30 mins of YouTube, then go back to your bank, and you will be logged out.
Caching (and compressing) the page in an inactive tab means you have something to go back to other then just a loading bar. 99% of the time this is fine because it's just going to reload a "read only" page. 1% of the time though the refresh causes some kind of server side reaction (like the bank site logging you out) and you get a noticeable different set of content (log in page instead of bank statement).
So for fast review, if a page was held active, just on another tab, the page would be in an invalid state because the javascript on the page could not run, and because certain CSR protections would be incorrect. If the page was not cached you would get a nice white blank page, and no content while the request was sent and received to the server. And if is was not compressed, you would end up with safari chewing up Massive amount of ram (like 4-5 gigs) because of how tabs are really just documents, etc. More then likely the ram issue would present as lots of disk trashing in other apps because safari would have all the memory used for disk buffers assigned to it's self.