Spudis says it better than I can, so I'll quote him
We know that a Mars mission staged entirely from Earth would require an enormous amount of mass to be launched; using a super heavy lift vehicle (such as the Ares V of the cancelled Constellation project), it would require between six and twelve launches to mount a single human Mars mission. Getting “millions” of colonists to the Red Planet on this basis would be cumbersome, to say the least.---Paul Spudis
But getting only part of the propellant you need, like oxygen from the Earth's upper atmosphere, can be cheaper than getting it all from the Moon. It costs fuel to deliver mass, so the less mass you have to deliver, the less fuel that it would take. Let's say you get your hydrogen from the Moon, and your oxygen from the Earth. Then, as a practical matter, it takes nothing from the Earth to get fuel to a Gateway at EML-2. If you bring back carbon from Mars, you can make methane, and that will work with Musk's engines. Or you could just mine the hydrogen from the Moon for a NTR rocket at EML-2, and cut your fuel budget even further. What this means in terms of significance, is that much more mass can be devoted to the mission than for fuel.
But the main point should be well-taken that to stage from Earth is a "cumbersome" project "to say the least".
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