Why did the family choose Grandpa Joe to accompany Charlie?

Since the ticket invited Charlie to take one or two members of his family to the factory with him, Grandpa Joe volunteers to go; they wonder if it is better for Mr. Bucket to go, but Mr. Bucket insists that Grandpa Joe deserves it most of all and is well enough to make it.

From the text:

‘There’s no must about it, my dear,’ Mr Bucket said gently. ‘Mind you, I’d love to go. It’ll be tremendously exciting. But on the other hand ... I believe that the person who really deserves to go most of all is Grandpa Joe himself. He seems to know more about it than we do. Provided, of course, that he feels well enough ...’