Check out http://en.wikipedia.org/wiki/Taxing_and_Spending_Clause for some interesting information on this in layman's terms. Looks like there wasn't an agreement on the reach of this clause by the founding fathers.
But under Supreme Court case law, Congress has pretty broad discretion in using its spending power.
As to your question:
Why would it bother to list in Article 1, Section 8 anything? Could they not all be found in "general welfare?"
No, they can't do anything under the general welfare clause. The Federal Government is bound by the enumerated powers in the Constitution. So while say forcing the states to do something might be beyond its power, it can entice the states to fall in line using the power of the purse strings. Of course there are some restrictions. See South Dakota v. Dole.