In the OP's example, evt will retain it's value if passed in as "truthy" or take on the value of window.event otherwise. The return then returns evt and evt.target or evt.srcElement which may from the original passed in evt or really from window.event, which was assigned to evt in the first line.

With your example, evt.target and evt.srcElement are only coming from the passed in evt which may have been passed in as "falsey" and would not take on the target or srcElement of window.event as intended.

I could be mistaken, it's late and I'm rusty. Anyone is free to correct me.