Manned exploration is expensive and risky. In general the science-per-dollar is much higher for unmanned probes. I won't claim on-premises analyses can't be helpful, but the ability of bots to survey more areas usually offsets that advantage. The real benefit of manned missions is learning how to live in space for future colonies and long-distance travel. Space is not easy on the body, we'll have to figure out how to adapt if we want to spread.