Once you upload something to the Internet…
Apart from giving you standard RSS reader functionality with a bit of social features built-in, Google Reader has at least two other interesting uses. Both are directly related to the fact that Google stores all contents of your RSS subscriptions in the cloud and loads up from there, for your convince.
First, it can serve amazingly well for browsing content on websites with troublesome navigation. You just get an endless scroll of RSS items, given the website does provide RSS with full entries and not just the headlines. The experience is kind of like Tumblr dashboard; only faster and more readable. Complements pretty damn well with readability, which is another tool I find myself using quite often.
Here’s the other one: since every feed loads from the cache, Google Reader also proves quite useful when websites go down or don’t exist any longer. Think of it as web.archive.org, but frequently updated and with contents dating back to 2005, which is when the service went online.
You Tumblrs don’t get your hopes too high on the second one though!
It doesn’t cache pictures.