Sobre o artigo ''A'': Como utilizar

Hello there, I'd like to know why in the first sentence of the following two sentences both “a” or “no article” can be written, but in the second only “a” is possible. By the way, I’ve adapted them myself in order to make the question appropriate for here.
He offered me a job as (a) road sweeper. He said that a road sweeper earns a great deal of money.