How are Mexican urban areas and Mexican rural areas interdependent? 
How and why do they need eachother?  Why cant Mexico be completely
agricultrual, or completely urban?

Forget Mexico

- go back to Europe and ask why towns exist

- even better go back to 'Ur' 
  the Mesopotanian 'town' that gives us the 'Ur' in Urban

What is different between Towns and ... rural areas ?

Ask yourself why people tend to gather in Hamlets, Villages, Towns, Cities.

Are there material advantages ?
- if so look at them.

Reverting, Mexico is not a closed economy, it's urban developments are
trading cheap labour with others, it is a bit like an upside-down
pyramid.

Also, look at mechanization .. and curiously those things that are
hard to mechanize.

Ask a more open ended question, why don't you?  People have written
whole doctoral theses on such matters (taking years), and you're
offering $20?  What's up with that?