Some people prefer 91 octane non-ethanol gas.  Some people want all the octane they can get and are willing to trade off fuel economy (or a wonky fuel gauge).

Husky has always offered ethanol blended gasoline for higher octane, and their 90 octane was made with 87 octane feedstock + ethanol.  For the price of regular gas it was a great performance bargain http://www.westernsubaruclub.com/smf/index.php?topic=9061.0 (and showed great tuning results: http://www.westernsubaruclub.com/smf/index.php?topic=20038.0 http://www.westernsubaruclub.com/smf/index.php?topic=2034.0)

However, with new legislation requiring an average 5% ethanol in the fuel at the pumps, H90 is no more as non-ethanol 87 feedstock is no longer being produced.

Thread about the demise of H90 here: http://www.westernsubaruclub.com/smf/index.php?topic=18684.msg280193#msg280193

Threads about Husky 94 vs. non-ethanol 91 octane here:

http://www.westernsubaruclub.com/smf/index.php?topic=4817.0
http://www.westernsubaruclub.com/smf/index.php?topic=13233.0
http://www.westernsubaruclub.com/smf/index.php?topic=633.0
http://www.westernsubaruclub.com/smf/index.php?topic=17419.0