Why? Because there is a great selection of fruits and veggies available at the crest of their flavor and nutritient value.  Fruits grown during their natural seasons tend to taste better and reach a greater nutrient density when compared to produce imported from other countries (or grown in green/hot houses), which often fail to ripen properly and leave an unpleasant taste and texture. 

What's in season right now?  A few popular fruits and veggies are...

Fruits:
Figs
Apples (starting)
Blackberries (ending)
Peaches
Grapes
Avocado

Vegetables:
Cucumbers
Summer Squash (ending)
Peas
Basil
Artichokes

(Check out these websites for lists of more seasonal vegetables and fruits.)

Also, eating local produce is a the greener of the two choices, requiring much less pollution created by shipping and handling than imported goods. 
In the fashion of Alton Brown, seasonal foods are simply "good eats".