A playwright would use a soliloquy instead of a monologue primarily to reveal the character’s true motivation and most honest feelings.
While both soliloquies and monologues involve a single character speaking at length, a soliloquy is typically delivered when the character is alone on stage or believes they are alone, allowing for a more intimate and genuine expression of their thoughts and emotions. This contrasts with a monologue, which can be addressed to other characters or the audience and may not necessarily reveal the character's inner thoughts in the same way.