Stargate uses the analogy of phone calls quite often, so I'll try to give an answer in that direction.
Imagine that phone calls within the United States are very cheap, and that phone calls within Canada are similarly cheap (disclaimer: I have no idea if any of this is true, but you may have guessed as much already).
Phone calls between United States and Canada, however, are so prohibitive that you can't afford to make any unless you really need to.
So what do you do? You build a house on the border and have two phones in it, one connected to the United States phone Network, the other to the Canadian network. So now you can make phone calls between the United States and Canada for a more reasonable price and you don't need international (8th chevron) dialing.
(That said, I think it doesn't really make much sense that a Milky Way to Pegasus wormhole would be so prohibitive if you can cover the same distance with a serieseries of shorter, cheap ones. I guess the telephone analogy hold there too, why do international calls need to be so expensive?)
The power requirements of dialing Earth from Atlantis required a ZPM, with the midway station, being you have a Milky way gate, and a Pegasus gate you can dial the MW one with the same amount of energy it would take to dial the Pegasus. Thus no ZPM required
