Why should you donate cards?

To add to the above and expand, and probably what is the best reason to donate is: You can get up to 4200 gold per week from donations. Explained here! If you are in a big clan (40-50/50) you will be able to always donate cards reach your daily limit, and yet keep the cards that are currently valuable to you.

For instance: as I constantly save up and don't level the common crap in my deck I have 15 different cards that I can level up, most of them more than once, AND I also have 5k gold on 1300 trophies. Most of that is from donation in my clan, where last week I had donated ~240 cards. I don't donate the one common and one rare I have in my deck, and donate everything else. I initially checked whether or not a player donated to the clan, but we recently had a good clean and all players are decent now.

If you donate a common card (up to two per request) you get 1 experience and 5 gold.

For a rare card donation, you'll get 10 experience and 50 gold.

Apart from that, many clans have a 'donate-or-leave'-policy. In the clan the number of weekly donations is visible, and no-donators can be kicked out, if your clan is that strict. Furthermore, the feel-good factor in a clan goes up when there are a lot of donations, possibly increasing the chance that your own requests will be granted.