I entered Item ID, Max Bid then clicked the "Add Item". (and nothing else).

Does it automatically put this into "active/ending" state?
I see there's a "Snipe" button in the table row. What does this do?

Think about it this way:
If you don't set your maximum snipe amount, how the sniper will know how much you want to bid?

Of course, you need to put your maximum you are willing to bid up to, otherwise it won't bid.