When someone is oblivious and blocking the aisle, and I want to get there attention so they'll move, just as one example.

Also pretty common when there is a collision and it isn't clear who is at fault.

It is a matter of politeness; much like please or thank you.

I say pardon or excuse me when I am about to pass by someone e.g., to let them know and not to be startled, e.g. when going behind them. Don't have to bump into someone to say excuse me.

Like a wave and a smile when another driver gives you space to make a turn.

I almost exclusively use "excuse me."

Since they have similar meanings, and I use them interchangeably.