Purpose of Steve and Eunice

We can see in "A Streetcar Named Desire" that the purpose of Steve and Eunice is to provide parallels for Stanley and Stella, We see that in both relationships there are abusive in aspects, consequently we see that Eunice throws Steve out where as Stanley and Stella have made peace by the end of the night due to their more primal and animalistic relationship which has a more physical basis to it. Steve and Eunice represent the life Stella has chosen for herself and Williams makes it seem, that this is possibly the time of life all people of this class have to go through and that due to Stella's choosing of it, it shows us that she has rebelled or gone against the life she was handed to and instead forsaken it for this "New American" class. Those who have to work instead of enjoying the life like Blanche did for so long.

So pulling from previous points I can infer that Williams presents this as being the norm at the time, A man would beat his wife yet would go back to him due to the stability he could provide, even if they had other options, this was the case for Stella originally. Furthermore due to the loss of Belle Reve she has nowhere else to run to, except Stanley. We see that it works for Stella and Stanley. However it may not work forever for Eunice and Steve, Blanche being appalled at the abuse she has witnessed and wishes to leave.

So to reiterate, The purpose of Eunice and Steve is to provide parallels for Stanley and Stella.

(Really had trouble writing this one, The two just appear as characters who act as a way to further the plot after the Poker Scene)