Washing at this point raises the question: why now?  Is it because slaves had no time to do it? Is it in gratitude for clean water?

Rabbi Elazar said that Rav Oshaya said: Anything that is dipped in a liquid before it is eaten requires the ritual of washing of hands, preventing people from making food ritually impure. Talmud Pesachim 115b

Wash this time without a blessing. Shulchan Aruch 158:4 

Pour cup of water over one hand three times then the other; pat dry with a nice towel.

haggadah Section: Urchatz