The monovalve controls the flow of hot water through the heater core. Most modern cars always have hot water flowing through the heater core, and use a door to direct some or all of the air around (or through) the heater core. The 124 takes a different tact - it regulates the quantity of hot water admitted to the heater core, allowing the push button unit to control the temperature of the heater core. Is this better than other designs? I dunno, but it seems to work OK. Each have their problems.
When the monovalve becomes leaky or sticky, it may fail to completely close off the flow of hot water to the heater core when commanded to do so. Hence you get lukewarm to hot air conditioning. (The heater can completely overpower the a/c.)