Nothing *needs* to be manned, just as the USA didn't have to go to the Moon. However a manned mission is more challenging, and as such it calls for development of technologies and vehicles that otherwise would be not developed here.
A manned mission is also more flexible. Imagine that you sent a probe to an asteroid that is flying through the system. The probe approaches, takes a photo, and you suddenly see an alien ship on that asteroid. What do you do with your probe? Nothing. You say goodbye to the FTL drive that is on the ship, since the asteroid won't be back until year 2387. But a manned mission most likely includes landing (which is very cheap there) and the crew can explore the object all they want.
But we don't even need to look at such exotic possibilities as an alien ship. You have a deep fissure in the asteroid, and if you can descend (which is again trivial in near zero gravity) you can see what the asteroid is made of, beneath the surface dust. A robot can't do that - they aren't smart enough yet to do such a thing. But an astronaut can easily go into such places, take samples, look around, follow clues and such.
In other words, a robot (as we know them today) can do only a fixed mission, and robots do them just fine. I would never send a human to stay in orbit around Mars for a year and take 12,000,000 photos of the surface. This is something a machine is better suited for. But if you need to investigate something then a robot can spend a year there and still not accomplish what is needed. As another example, the robot on Mars can dig a trench up to 20" deep in hope of finding water ice there. What if the ice is deeper by just 1"? A human would dig as deep as practical, climbing down into the hole as needed or making an extension for the shovel out of spare parts and duct tape. A robot can't dig deeper than its mechanical arm is, end of story. Or another example. A robot can drive for a day to a rock, then grind a stone a bit to see what's under the surface. A human would take a hammer and split the rock in two, and the answer is immediately obvious.
Haven't we already developed the technologies for manned missions?
But we don't even need to look at such exotic possibilities as an alien ship.
Then why did you mention it?
You have a deep fissure in the asteroid,
Do asteroids have deep fissures, and if they do would NASA risk sending an astronaut into it?
A human would dig as deep as practical, climbing down into the hole as needed or making an extension for the shovel out of spare parts and duct tape. A robot can't dig deeper than its mechanical arm is, end of story.
How deep did the astronauts dig on the Moon?
A human would take a hammer and split the rock in two, and the answer is immediately obvious.
Why can't the machine return the rocks to Earth to be studied?