Voice types in opera often differ to those in musical theatre, using much more vibrato – and are often much louder. This is because of the long history of the form, which was performed before microphones were created, so opera singers had to project over the orchestra without amplification.
Musical theatre is a comparably newer art form, so its singers are often given microphones to help them be heard over a loud band or orchestra.
Vibrato has the effect of warming up the voice and helping it carry over a large orchestra and across a concert hall. It shifts the pitch and frequency of a note ever so slightly, but happens so quickly it is barely noticed. As a result, singers can achieve real volume and reach the back of a concert hall with their loud voices.
Vibrato is also used by string and wind instruments.