Like the name says, an operating engineer is in charge of keeping existing equipment running and in good repair, as opposed to designing new equipment.
There is an unfortunate tendency to rank operating engineers lower than design engineers. One good antidote is to consider perhaps the most famous (albeit fictional) operating engineer of recent times, namely Star Trek's Scottie. Scottie did not design Starship Enterprise, instead he keeps it running. Not infrequently, running in ways its designers never intended.
Increasing automation has greatly reduced the number of operating engineers needed to tend buildings, as boilers, air conditioners, and elevators are much more able to fend for themselves than were their decades-ago counterparts.
In fact, the most “needy” mechanisms in modern buildings are normally computers. These do have operating engineers, but we call them “system administrators” (or network, database, etc. administrators). These “administrators” are starting to gain the same kind of grudging and backhanded respect that is traditionally accorded operating engineers.
Which is just as well, given that most people will notice the absence of an operating engineer much more quickly than that of a design engineer.