As Dr. Malt said, the rates of chemical reactions increase at higher
temperatures. Rates roughly double for every 10 C or 18F. Brewers
want the mashing process to go as fast as possible, so they mash
at as high a temp as possible. Above about 170 the proteins unfold
(denature) irreversibly, otherwise you'd mash close to the boiling
point of water to complete the process as fast as possible. The
purpose of the enzyme in the organism is to break down the starch
in the seed to provide fuel for growth, and this process doesn't
happen in 30 minutes or an hour as in a mash, but over several days
at the temperature of the ground.
Related to this is the rate of fermentation. I constantly see remarks
in this newsgroup about how great the yeast was, the fermentation
took off like a rocket etc. but that's not what you want. You want
a slow contained fermentation to minimize byproducts. The rates
of byproduct fermentation increase at higher temperatures just
like the rate of alcohol production. If your fementation blew out of
the carboy, the temperature is probably too high.
Ask not for whom the beer twangs; it twangs for thee.