Use when you're genuinely unconcerned about choices and want to show willingness to comply with others' preferences. Choose any restaurant, I'm easy.
Pro Tip 2/3
Avoiding Responsibility
It can imply you're avoiding making decisions and are leaving it up to someone else. You pick the movie, I'm easy.
Pro Tip 3/3
False Consensus
Don't assume saying 'I'm easy' always keeps things simple; it may pressure others to choose for you. He said, 'I'm easy', yet seemed disappointed with my choice.