If there is nothing better, useful or interesting to do, you use 'may as well'. ''All the pubs are closing. We may as well go home".

It can be the answer to a question as well. "Are you going to start your Christmas wrapping tonight?" "Yeah, might as well."

La expresión significa aprovechar un situación, buena o mala, y no hay razones que no hagas alguna cosa. Ejemplos: It is rainy outside. I may as well stay inside and watch a movie It is sunny outside. I may as well go for a walk. Se usa "might" por "may" en la misma manera. It is rainy outside. I might as well stay inside and watch a movie It is sunny outside. I might as well go for a walk. De hecho, pienso que "might as well" es más común que "may as well." Técnicamente, en respecto de gramática, tal vez haya una diferencia entre "may" y "might" acá, pero no creo que muchos hablantes nativos de inglés prestan atención a estas diferencias.