There's so much information that the Internet can give us. I can look up whatever I want at anytime as long as I have WiFi or 3G. I don't have to wait until I'm near an encyclopedia or a computer anymore.
I remember looking up words for my word study class in elementary school. I would learn random words along the way. I learned the word "contortionist" while I was looking up another word that started with C because there was a picture of one that caught my eye. dictionary.com doesn't provide that opportunity, but it provides me with what I want to know at the exact time I want it. When I was a senior in high school, my English teacher made it mandatory for us to use a printed dictionary and not an online one.
So while online dictionaries help us learn because we can look up more words in less time before we forget that we wanted to look up the word, it also blocks out some of the other things we can learn from using printed dictionaries. There's also no way for a printed dictionary to be unreliable, like Urban Dictionary, or for it to be covered with distracting spam that does nothing for your brain. The downside to printed dictionaries is that they can quickly become outdated.
The other side of this is that we choose what we want to read in print, but not always what we read online. For example, sometimes I feel like posts like these make me dumber, and I don't exactly go looking for it. I feel bad for the people who don't know that they made spelling errors before they're broadcasted to the world, but I also feel bad for the people who are connected to them on social media who don't know that these are errors.