The earth doesn't have an unlimited supply of resources. Asteroids are literally loaded with minerals. At some point we are going to deplete the earth and will have to start looking to the skies to get what we need.
I'm not convinced that asteroid mining would be more productive than, say, tapping into rift zones, but it's an interesting problem. Might be useful to solve.
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Notes:
1. https://en.wikipedia.org/wiki/Asteroid_belt#Characteristics http://iau-comm4.jpl.nasa.gov/EPM2004.pdf
So until we'll be able in practice to make everything out of electricity, dust and the power of will, we need various resources for our survival and growth.