why do people usa a different pen for each signiture when singing treatys or offical documents?
I usually have my assistant use my personal stamp, 
Traditionally the pens used to sign laws and treaties are given out as souvenirs. The more pens you use, the more there are available to be given out to the bill’s sponsors, lobbying organizations, etc.
When a President or governor signs what he considers a piece of landmark, historic legislation, he likes to make a big deal of it. There are several ways he may do this.
Suppose, for example, President Bush is about to sign a major piece of tax-cutting legislation. He may make a point of signing it with a pen that once belonged to Ronald Reagan. Or, when Bill Clinton signed a piece of job-creating legislation, he may have signed it with a pen that Franklin Roosevelt once used to sign an important New Deal-era bill.
If a President really believes that a piece of legislation he’s signing is of major, historical importance, he may sign it with several different pens, and then give those gifts to various people or institutions.
Let’s say (PURELY for the sake of argument- I relize this is unlikely to happen) that Congress passes a hate crimes bill and George W. Bush is going to sign it. He might use 11 different pens to sign his name. He might then give one of the pens to the family of James Byrd, one to the family of Matthew Shepard, one to the family of Medgar Evers, one to the Smithsonian Institute… and so on, and so on.
This sort of thing makes for good PR and good photo opportunities.
For souvenirs. It provides a keepsake to folks who were involved in getting the treaty/law/whatever done, political allies, distinguished guests, etc.
Or, you can just read what Astorian said. He owes me a souvenir electron for beating me to the punch.