An MIT order is the mirror image of a stop-order. A buy stop sits above the market and triggers on strength; a buy MIT sits below the market and triggers on weakness. Both become market-orders at the trigger.
It exists because traders want automation for entries they would otherwise watch for. It is not a protective order, and like any market order it can fill far from the trigger in a fast-market.
Example: crude is trading 78.40 and you want in on a dip to 78.00. A buy MIT at 78.00 fires the moment 78.00 prints, filling at roughly 78.00 to 78.02. A buy stop at 78.00 would never trigger, because price is already above it.
Related: limit-if-touched