I still buy Merriam-Websters Collegiate Dictionary and give volumes away to friends' children when they start secondary school - and they thank me a year or so later! There's something about the way a dictionary is organised that makes it a better learning tool than just seeing the word in isolation online (which is fine for just grabbing the gist). When you see the long entry and the related words surrounding it, it makes truly understanding the word easier and therefore remembering it.
I like Websters over the condensed OED because although American it not only gives British spellings and usage, but also a very good amount of Scots. It also assumes that the owner will read Shakespeare, so the words you are mostly likely to look up as you read the bard are in it. It also contains other useful information like international phonetic script, punctuation guides, how to cite references, and how to address dignitaries, and the like. Sometimes it's great to be able to quickly look that up without being distracted by the internet. This is great if you're on of those people who has to block the internet from yourself to stop procrastinating and suddenly need to look something up.
Also, it's heavy and hardback. You can flatten things under it, press things between its pages, sit on it if you're short, balance it on your head to work on your posture, and lob it at intruders. It aslo looks like a spellbook as a prop if you take the dust jacket off.