I am trying to think of what the House Manager does... Let the dog out every once in a while? Open the door when Comcast comes? Pay both the nannies on time?
I used to work in very high-end home construction and interacted with Household Managers. A lot of people from my field move into that job, because they get to know a family through the building process, earn their trust, and get an offer as the home construction winds down. Basically, a Household Manager makes sure all the moving parts in the house are running. They find and preliminary interview all staff (nannies, tutors, landscapers, pool guy, the person who services your bowling lane, the personal chef, the driver taking the kids to school). They pay the normal household bills from the operating accounts (check the statement on the nanny's credit card and pay the bill, pay the electric, pay the plumbers). They ensure the household inventory is always there (is the boss' favorite drink always at hand in the fridge? Is there enough wine on hand for the dinner party next Friday, or should I call our wine guy?). They know enough to ensure that repairs are completed correctly and can solicit and select among bids for contractors. They are often capable of performing minor household repairs. They ensure the family vehicles are always full of gas, clean and in excellent working order with routine maintenance performed. They make sure the family's technology is always in good working order. When the boss's teenaged son is having a sleepover, the household manager makes sure all the video game systems are working, the pantry is fully stocked and there's sufficient clean and pressed bedding.
None of this is rocket science, but generally, the people you are dealing with have very demanding standards. We're all willing to tolerate a poorly draining tub until the weekend when we can get to it; they aren't. They have a big expensive household and lifestyle full of Expensive Fun Things and they want them all to work perfectly all the time. Often these people are not idle rich, but very busy executives or business owners who don't want to dick around fixing a slow computer; they just want it to work perfectly and immediately, so they can spend their scarce free time with their wife, kids and friends.
Household managers also dealing with houses that are much more complex to clean, repair, maintain than your average three bedroom ranch. We built a spec home that had, among other things, an ice cream parlor, a water slide, a full sized basketball gym, a bowling alley, two guest houses, two sleepover rooms. All the furniture was custom and much of the hardware, appliances and household equipment were imported from Europe and there were limited people available to fix it. It's actually a really interesting line of work to someone like me, and one I probably would have transitioned into, had I stayed in the industry.