In my classes only about 1% of the dictionaries my students use are printed on paper. I have talked to people working in bookstores and they confirm that dictionary sales have fallen. Online dictionaries are easy to use; the word in question is just typed in. The user can listen to the pronunciation instead of using the IPA. Why should students pay for a dictionary when some are available online for free?
Earlier I wrote an article giving advice on how to choose a dictionary. That advice also applies to online dictionaries. However I would like to offer some additional advice.
Students have asked me for help when their online dictionary gave a strange pronunciation. Not all online dictionaries are the same. Compare the pronunciation they provide with other reliable sources. Certain words should show two (or more) pronunciations to account for American, British and other local English varieties. Look up the words vase, herb and trousers.
Some online dictionaries are based on old dictionaries whose copyright has expired. There often have been relatively few modern additions. Check to see if they offer definitions for common, but recent words: laptop, hippie, email, stagflation, online, motel...
Many English words have changed meaning over the centuries. The last hundred years is no exception. Your dictionary should have the more recent meaning. Better academic dictionaries will also have the older, archaic, meaning.
1. (archaic) a casting or throwing of seed in all directions, as from the hand in sowing.
2. (modern) to send out messages or programmes to be received by radios or televisions
1. (archaic) exists only in the imagination, not real
2. (modern) very good or wonderful
1. (archaic) happy
2. (archaic) brightly coloured
3. (archaic) sexually permissive
4. (modern) homosexual (also a noun)
I grew up surrounded by books and I still like browsing through a paper dictionary. Few people have ever read a dictionary cover to cover. (I have, but I do not recommend it as a way to learn a language.) I can look through its pages and many other words catch my eye. One word reminds me of another or shows me a new word. However that is a matter of what a person is accustomed to. Dictionaries are valuable references when learning a language.
I do not want to recommend any one online dictionary over another. However the site www.onelook.com allows the reader to compare many online dictionaries quickly.