Just read this article last week! Interesting stuff.
Return of the Fullback Part III
The spread offense was popularized with the rise of the singleback offense, which basically replaced the fullback with a slot receiver. However, teams learned that finding dual-threat tight ends was pretty difficult and then they further learned that having a blocker off the ball could have value in a spread concept even though the team could no longer run “four-verticals” with only three WRs on the line.
First off in terms of the two-back run game, being able to insert a lead blocker at the point of attack has certain advantages that you don’t get when you have three WRs and a TE on the field. Perhaps more importantly though, the “smashmouth spread” is based around the tremendous advantage that you can get from forcing a defense to account for two potential ball carriers each working behind a lead block.
The H-back alignment is perhaps a little more common, as is the TE/H-back de-facto FB, because they’re often big enough to use the couple of steps to knock back a DE on a kick out or trap block.