A simple question, so to re-iterate the title:
Why is a "field" on a form called a field?
A quick search for the etymology results in:
Old English feld "plain, open land" (as opposed to woodland), also "a parcel of land marked off and used for pasture or tillage," probably related to Old English folde "earth, land," from Proto-Germanic *felthuz "flat land" (common West Germanic, cf. Old Saxon and Old Frisian feld "field," Old Saxon folda "earth," Middle Dutch velt, Dutch veld Old High German felt, German Feld "field," but not found outside it; Swedish fält, Danish felt are borrowed from German), from PIE *pel(e)-tu-, from root *pele- (2) "flat, to spread" (see plane (n.1)).
Finnish pelto "field" is believed to have been adapted from Proto-Germanic. The English spelling with -ie- probably is the work of Anglo-French scribes (cf. brief, piece). Collective use for "all engaged in a sport" (or, in horseracing, all but the favorite) is 1742; play the field "avoid commitment" (1936) is from notion of gamblers betting on other horses than the favorite. Field glasses attested by 1836.
No mention of its use in forms?