Many trading strategies operate on very thin margins. Most of the time it's less than one cent per share, often as little as a tenth of a cent per share or less.
A different example: let's say that you're trying to buy some security, and you've determined that the maximum price you can pay and still be profitable is 10.01. If you mistakenly use an order price of 10.00, you'll probably get fewer shares than you wanted, possibly none. If you mistakenly use a price of 10.02, you may end up paying too much and then that trade ends up not being profitable. If you use a price of 10.0199999 (assuming it's even possible to represent such a price via whatever protocol you're using), either your broker or the exchange will likely reject the order for having an invalid price.
A different example: let's say that you're trying to buy some security, and you've determined that the maximum price you can pay and still be profitable is 10.01. If you mistakenly use an order price of 10.00, you'll probably get fewer shares than you wanted, possibly none. If you mistakenly use a price of 10.02, you may end up paying too much and then that trade ends up not being profitable. If you use a price of 10.0199999 (assuming it's even possible to represent such a price via whatever protocol you're using), either your broker or the exchange will likely reject the order for having an invalid price.