As we see in Act III scene 2, which is when Ganymede first meets Orlando, adopting a disguise, particularly if you adopt a different gender, allows you to relate to characters in a completely different way. Consider what Rosalind says as she prepares herself to meet Orlando in her disguise as Ganymede:
I will speak to him like a saucy lackey and under that habit play the knave with him.
Disguise therefore is something that can be seen as opening up a whole wealth of possibilities and benefits: Rosalind is able to act like a man, which in a male dominated world, gives her the freedom to play with and deceive the man that she loves. As we go on to see, Rosalind uses her stratagem to ironicaly assume the identity of the person Orlando loves--Rosalind herself--in order to discover more of his character and to train him in how he should express his love. With Rosalind-pretending-to-be-Ganymede-pretending-to-be-Rosalind we see the true benefits of disguise.