http://www.homebrewersassociation.org/category/recent-alerts/
Gives all the recent and ongoing issues. May not make any difference where you may live, but overall, I want everyone's ability to brew to be unhampered as much as possible.
Last year the California ABC interpreted a then new law that was intended to make homemade wine legal to donate to charity (a good intention), as making it illegal to donate home brew to a brew festival. As a result (i.e. unable to get permits), the Southern California Homebrew Festival was cancelled for the first time in 24 years. Gray Glass and the AHA were very helpful in our lobbying for a new law (which was passed) to fix this problem. The festival is back on this year (now at Vail Lake Resort). Just one of many examples of the good work AHA does.
That is the biggest "Why?" for me. I don't live where I get discounts at pubs, and there aren't any AHA rallies close by- but I am a big proponent on supporting the AHA's efforts.
There are other things too- like grants for the Research and Education fund, which can help all of us be better brewers.
That's the main reason I'm running for the AHA Governing Committee. (The elections will start later this month). I think that we ALL benefit from the work it does.