......I suspect we’d find rocks....
The fact is, we don’t know what is to be found. The term rocks is being used as anything that is solid. What is important is more specific, the compounds that make up the solid.
There has been a lot written on the subject but I recommend “The Moon Is a Harsh Mistress” by Robert Heinlein. He explores concepts of moving stuff about.
To many the concept of living with uncertainty is intolerable. For others the uncertainty of what lies ahead is what makes life worthwhile. Uncertainty is the reason for exploration at Yellowstone, in a cave, under water or in space.
As to learning about what's on Mars, if we accept that at most ten or twelve people will go there at great expense and come back home, what is the value to us here on earth? The early explorers went for gold, for silk, for spices, and sometimes to build a new life in a strange land. That's not here. Not in space exploration. In space exploration we've got the desire to explore and scientific and intellectual curiosity. I don't think that is enough to sustain the level of effort required to do what we'd need to do to reach out beyond Mars at the most, except for the stray unmanned satellite every few years.