Although others can provide some more detailed explanations if they want, a short answer is just "Because it works."
The ultimate rationale behind cut blocking is no different than that of tackling ballcarriers low. If you go low, the target has a harder time staying on his feet (and, thus, making the tackle or running for a touchdown).
On the backside of running plays, it's usually the most sure way to get across a defender's body and prevent him from chasing down a play. Zone teams often/usually cut on the backside too.
With quick-hitting option plays, you don't have to keep a defensive lineman blocked for an extended period of time, so a quick cutblock which can prevent a guy from making a tackle within the first 1.5 seconds is a successful one. If you are zone-blocking a guy, waiting for a running back to choose his lane, you don't have that option; a defender who is cut can get back up and fill a gap in that longer time frame. Furthermore, zone-blocking is based on combination blocks between two defenders, making the cut block illegal in most situations.
That being said, we don't always cut block, not even on our triple option play.
Notice the right guard engage the DT just as he would in any other scheme.
A lot of this is situational - where is the defender lined up, what's the play call, what has been working, am I likely to get help with this block, etc?