Ingredients are cheaper than when brewing with extract, because of the lower-cost of bulk, whole grains. The cost savings are offset by the extended brew-day that the technique requires.
Ingredient flexibility and freedom by incorporating the mash brewers are able to customize their recipes by utilizing grains that must be mashed, for which there is no extract available. (Smoked Malt, Vienna, Brown, Mild, Amber, Aromatic, Biscuit, etc.)
Familiarization with Mashing, Sparging, & Lautering bring brewers closer to exploring the world of full-scale all-grain techniques.