. . is why you fry the flour in butter to make a roux.

In the traditional French Cuisine, when making up a big batch of a basic brown or white sauce the roux is stirred into the sauce. The sauce would then be simmered very slowly during which time the thickening starch would dissolve and the non-soluble parts of the flour would slowly form a scum on top to be skimmed off by a watchful kitchen boy (today that would be an illegal from Guatemala).

Escoffier stated in his books that in the future pure starches would be used to eliminate the costly skimming, but since he stuck to traditional methods, so did his followers.

Pure starches need to be selected with care - some produce a shiny "cheap Chinese restaurant" effect, others don't.