All the back and forth emails between the two during Shaun and the crew's time in Italy helps Shaun get a better understanding of the man - and a sort of companionship outside and in the assassin life. Meeting him in person gives him an even better opportunity.

+If William is *not* a replacement or consolation prize for Shaun not getting Desmond.
++If there's stepdad jokes thrown in at Desmond's expense.