It can mean "I put in unnecessary effort."
"To get to New York in time for the meeting, I paid $250 for tickets on the high-speed Acela train; then I found the meeting was rescheduled for late in the day, and I could just has well have bought cheap tickets on the regular train."
It can mean "I _didn't_ put in extra effort, and, by pure luck, I didn't need to."
"It was just as well I didn't buy those Acela tickets, because the meeting was cancelled."
It can mean "Two unequal things work equally well for your purpose."
"Do you really need a $5,000 Rolex when a $20 Timex will tell the time just as well?"
"Of course the kids enjoyed Walt Disney World, but, honestly, I think they'd have liked any amusement park just as well."