Typical OL splits are 1-2 feet wide (generally tighter on the interior, getting to be slightly wider on the edges). However, in the spread the OL splits are usually 3 feet wide or sometimes more, which is the basis for your question. Based on my experience, there are several factors for the wide splits on the spread offense. By widening out the defensive front, you create wider passing lines that don't require a QB to be 6' 5" in order to find them, as well as opening the middle of the field for a passing attack by forcing the DEs to keep containment on the QB/RB by lining up on the outside shoulder of an OT that has taken this wide split. Just as it creates wider passing lanes, it can also create wider running lanes for a RB on the sometimes rare draw play or inside zone by spreading out the defense to account for more turf. As an OL it helps make pass blocking easier. Almost every defense tends to make adjustments to their alignment for blitzes to make their success more likely - either by changing parts of their stance or their physical alignment on the LOS. An experienced OL can pick up on these changes and know what to expect based off studying the opponent prior to the game, thus in the spread these changes are more easily noticed because of the wide alignment of the OL. In general, the wide splits simply make it farther for a DL to go in order to get to the QB. Specifically as an OT, the wider alignment makes you job easier, as it cuts off the arc that a speed rusher could take outside of you to get to the QB. This allows for an OT to get into proper position to cut the defender off from the QB easier and quicker, thus allowing him to concentrate on the technique for the block. Also given the wide splits, the OT doesn't have to go as far to reach this cutoff point. As an explanation of the cutoff point, imagine a straight line from the DE to the QB; the wider the OL splits out, the more directly this line will go through the OT, meaning he is already in better position to protect the QB; the tighter the OL splits, the more he can run upfield around the OT to get the the QB, meaning the OT has to more farther and quicker than with the wider splits. Given that the DEs are one of the most athletic positions in today's game (ie: Carlos Hall - 6' 4" 265 lbs, 4.4/40, 450 lb bench press - also Jevon Kearse, etc), you want to give your OT every possible chance to succeed against a superior athlete.
The down sides to the wide alignment are that you are sometimes susceptible to a well timed blitz - especially up the middle. If a LB can time out his blitz well, he can be right at the LOS with a full head of steam at the snap of the ball (here is a great case for altering the snap count and a smart QB baiting him into an offsides call). At the snap, the interior OL has to slide to account for this blitz, or depending on the protection scheme, the RB has to pick him up. In either case it is alot to ask of someone that is already at a disadvantage - the OL mismatched on speed, the RB mismatched on size, strength, etc. In the running game, the wider splits vitually limit most of your options to the middle of the offensive set because the combination of lack of quickness and speed on the interior OL and superior speed and width of the DEs. The extra width on the interior splits can create additional area that an OL has to cover on a combination zone block that can allow a good DL to split the block and create havoc. Of course there are exceptions, such as the occassional shovel pass where the DE is baiting into rushing upfield, thus creating a void behind him and a running lane for the RB.
