Why does the Torah mention the fact that only offered to draw water for the camels after she had finished giving him to drink?
Seforno: Because the Gemara says in Ta'anis (5b) that 'One does not talk during a Se'udah, in case the food enters the wind-pipe instead of the esophagus.'
Why did she say "I will draw water for the camels", and not "I will give them to drink"?
Hadar Zekenim #1: It is not proper for a woman to feed male animals.
Hadar Zekenim #2: The camels drank from a trough into which she drew water. She needed only to draw it.