mars probe?

Question

what could we hope to gain by sending a probe to mars

Answer 1

NASA has a phrase they use a lot. It is, "doing good science". It is their justification for the billions of dollars they spend on space exploration. .
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What NASA does best is justify it's own existence with talk of "doing good science". It's up to the tax paying public to decide how much it wants to spend on scientific research.
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When questioned as to whether or not humans in space are worth the cost NASA will always justify it with it's "doing good science" argument.
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Few know that after the Apollo project, NASA might have been dissolved and manned space flight might have been discontinued completely in favor or automated space probes.
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The decision to build the Space Shuttle made it through congress by one vote. The United States came that close to not having a manned space program after the Apollo program ended. Nixon knew the Lunar landing program was and would always be associated with Kennedy and Johnson who he despised*.
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The success of the Mars rovers of late have had the same effect of the general public questioning the need for sending men into space.
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Answer 2

Increased knowledge of the conditions and characteristics of the planet, deriving it's history, etc.

We currently have two rovers that are still sending data from Mars (Spirit and Opportunity). They have lasted well beyond their expected lifetimes.