Theirs was an unlikely coupling (a bit of a theme for the series), she being the Manhattanite lawyer and he the high-school-educated bartender from Brooklyn.
One of the funnier storylines of the series was centred around Miranda becoming pregnant to Steve. That was as unlikely a scenario as possible. She with the lazy ovary, he with only one testicle (the other lost to cancer), their relationship status in 'off' mode meant that conceiving a child was... well, inconceivable.
Steve's cancer crisis brought them together. But though officially they were only friends, Miranda took her role as support person very seriously. When Steve voiced doubts over his ability to attract women now that he was only 'half a man', Miranda kindly offered her services. Samantha called it a 'mercy f***'.
Hence, Brady Hobbs was conceived.
The Miranda/Steve storyline continued to entertain right through to the end, and provided some interesting character growth for both protagonists. Steve softened Miranda's hard line personality to the extent that she allowed his mother to move in with them when she became ill - a very 'big' gesture for Miranda the control freak.