Why was the Gadsden Purchase considered necessary for the US?

Answer 1

It was not the whole US that considered it necessary -- only the South.

There was at that time a great debate going on about plans for building a transcontinental railroad. Every group wished for the railroad to be built through THEIR territory, and to favor them economically. But the best proposal for a SOUTHERN route would not work UNLESS the U.S. could obtain that particular territory.

Fortunately for the South, recently elected President Franklin Pierce handed the matter over to his Secretary of War, Mississippian Jefferson Davis, a strong advocate of the Southern route (perhaps esp because he was grieving over the tragic death of his young son),. Davis sent Gadsden (U.S. minister to Mexico) to negotiate for the purchase from Mexico.

Side note: this is one of those episodes that undercuts the Southern "states rights" claim, that is, the idea that the concern of Southern leaders was simply for each . On the contrary, they often fought for national laws, policies and expenditures that would favor Southern REGIONAL interests over the North. (Other key examples --demands that the federal government force the Northern states to do more to protect slavery, enforce the federal fugitive slave law, etc.)
http://www.u-s-history.com/pages/h82.html

For more on Davis and the Gadsden Purchase (and the Southern concern with California at this time), see *The California Gold Rush and the Coming of the Civil War*by Leonard L. Richards (2007).