question

I don't understand when we use progressive instead of a long form like:
I saw them walking the street along. (Instead of 'I saw them how they were walking the street along')
There are two guys looking for a job. (instead of 'There are two guys who are looking for a job')
They caught him dealing with drugs. (Instead of 'They caught him when he was dealing with drugs.)

Thx,
moritz