Excellent question and I've wondered that myself. Without wanting to sound callous, I refer you to the phrase "the world needs ditchdiggers too." And no matter how technologically advanced we become, I believe the world will always need common laborers.
That said, I want to make it clear that I do not intend "common laborers" to be a pejorative term at all. There is a certain dignity in a simple job well done, be it painting a house, landscaping a lawn, or performing maintenance on an automobile. I'm not sure we will ever have robots perform those (and many other) menial tasks anytime soon.
In recent years, I have begun to hire people to do things around the house I used to do myself. For instance, I have my landscaping done by others, I pay an independent businessman to come plow and shovel my driveway everytime it snows. I also hire a husband/wife team to come clean my house every two weeks and have a handyman that comes periodically to do things around the house like hang ceiling fans or repair/paint window casings. All of this I could do myself if I wanted to save the money but I feel good about helping them make a living. Also, they tend to do these things way better than I would.
I think if everybody who could afford to do so took that approach, we'd have plenty to do for everybody.
Which is what illegals are for.