Why are chickpeas sprouted before grinding them into flour?
You don't need to sprout your chickpeas, but there are benefits when you do, as sprouting substantially increases nutritional value. By allowing the legume (or seed or grain) to germinate, the phytic acid within it is neutralized, as are enzyme inhibitors.

The Nourishing Gourmet Kimi Harris describes: 


  Phytic acid binds with calcium, magnesium, iron, copper, and zinc, making it hard to impossible for you to absorb those nutrients.  It’s also irritating to your digestive system. By sprouting your grains, legumes or seeds, you are neutralizing phytic acid very effectively. You will also be neutralizing enzyme inhibitors, which unfortunately not only inhibit enzymes in the actual seed, but can also inhibit your own valuable enzymes once they have been eaten.
  Beyond even anti-nutrients that are neutralized by sprouting, there are other changes that take place during sprouting that make it easier for us to digest our seeds/legumes/grains.


In Nourishing Traditions, Sally Fallon writes that:


  The process of germination not only produces vitamin C, but also changes the composition of grains and seeds in numerous beneficial ways. Sprouting increases vitamin B content, especially B2, B5, and B6. Carotene increases dramatically-sometimes even eightfold.


Sprout People offers comprehensive nutrition information and a chart listing the proteins, vitamins, amino acids, and minerals.

As a side note, while chickpeas are gluten free, sprouting grains that do contain it breaks down the gluten, a benefit to those with gluten intolerance.