Why was Marvel flabbergasted?

In chapter 9 of The Invisible Man, the old tramp Thomas Marvel is sitting with his feet in a ditch by the side of a road. He's trying on a pair of ugly old boots—charity boots, as he contemptuously describes them—when he suddenly hears a strange voice behind him:

"They're boots, anyhow," said the voice.

Marvel naturally assumes that there's someone behind him and continues to engage him in conversation, mainly on the subject of boots. After a short while, the tramp turns round to see what kind of boots the stranger's wearing. But there's no one there; no one at all. At first, Marvel thinks he might be hallucinating, but Griffin—for it is he—wishes to prove that he's perfectly real. So he starts throwing stones at the hapless tramp. Marvel recoils in pain, now ready to accept that he hasn't just been imagining things. Griffin tells him that he's an invisible man and sets out to prove it. Marvel reaches out and is astonished to make out the features of a man right in front of him, though he still can't see a thing.
Griffin explains to the tramp that he needs him to get clothing, food, and shelter. But Marvel is so flabbergasted by everything that's just happened that he can't think straight. It's all so terribly overwhelming. But as Marvel soon discovers, he's going to have to pull himself together pretty quickly, because Griffin isn't a man who takes no for an answer.