Question

Why did Portia disguised herself as Dr. Balthazar.

Answer 1

Well their are two main points why Portia disguised herself as Dr. Balthazar
1) As it is women's nature that they cannot believe the men in love it may be that Portia wanted to test the loyalty of her husband Bassanio by being with him without knowing Bassanio her real identity.
2) Secondly she wanted to free Bassanio's friend Antonio from the penalty of the death bond .

Answer 2

Portia disguised herself as Dr.Balthazar as :
Portia and Nerisa were going to Venice to help Bassanjo and his friend Antonio about the issue of Shylocks bond