The order validity defines when your order gets discarded if it is not yet executed. Why would it not yet be executed?
There could have been nobody willing to trade (that happens with small volume stuff)
The limit price was not yet reached
For example, let's assume, you giving a sell order for your Apple shares, with a limit price of 5000. As 5000 is not reached anytime soon, the order gets discarded after 1 day/end of month/whatever Validity you give.
Another example: You want to sell your 100 shares of 'Antarctic Gold Mines, Inc' for any price (=no limit). Nobody wants to buy them so the order sits around, until it gets canceled after the Validity chosen.
The Validity must be given, as the traders don't want an order sit in their books for the next 1000 years. Typically, the maximum you can chose is end of month, then all pending orders get cleared/discarded, but that might be different for different traders.
For a high volume shares offered without a limit, it typically doesn't matter, as there is always someone trading, so it goes through within seconds.