"In old analog cameras, many such filter “effects” were a chemical byproduct of the film, so photographers became expert at understanding the unique powers of each. Fujifilm’s Velvia film, with its high saturation and strong contrast, attracts photographers looking to capture the vibrancy of nature, Instagram cofounder Kevin Systrom notes. But casual photographers rarely developed this type of eye, because they just wanted to point and shoot. What Instagram is doing—along with the myriad other photo apps that have recently emerged—is giving newbies a way to develop deeper visual literacy." The argument for filters.

Citing the three main reasons as "Quality, Audience, & Constraints", Nate Bolt's piece on Instagram is worth a look.

"The days before Christmas, friends were sharing photos of the build-up, putting up the tree, wrapping presents, sitting on trains getting to parents houses, cooking hams. To me there was a real sense of flow, connection, joining in of everyone’s experience, a bit like a pictorial version of twitter to some degree. Christmas morning was almost magical.
"The previous year on Flickr was almost as magical, once people had a chance to sort through the 100s of photos they’d taken with their dSLR, pick out the best ones, run them through lightroom and then get a chance to upload them. It was nice to look back on the Christmas mornings that people had, rather than having."