Next to Full House, Family Matters is the best remembered sitcom of the T.G.I.F. era (with all due respect to Boy Meets World). Steve Urkel was a cultural phenomenon; the character got commercials, talking dolls, and everything else that comes with pop culture ubiquity.
The show was supposed to be a sitcom about a working class African-American family trying to get by, or, as advertisers might put it, quickly cancelled. The wacky neighbor became a fan favorite as wacky neighbors sometimes do, and that fan favorite became a phenomenon. Though Urkel remains one of the most annoying characters in television history, if Family Matters hadn't shifted focus, the show would've likely suffered a fate worse than Step By Step.