answer: to ensure that a seeker is sincere and determined. Jews have ALWAYS accepted converts, even when it meant a death sentence for the convert, the rabbi converting them and sometimes the entire Jewish community from their Christian or Muslim neighbors. Ruth in the Tanakh/OT is a famous convert (without whom there never would have been King David, Solomon, etc).
There is more to converting than just announcing you believe what Jews believe and you're set. It takes a minimum of a year to convert because there is a lot to learn to be able to participate in the community fully and in the religion. Seeing if someone is sincere or easily dissuaded helps the rabbi determine if the potential convert is going to stick through the long process and if this is where they are called.