If you grind at night, or you've had orthodontic work you want to keep, a custom appliance is a small thing that saves you from bigger problems later.
A nightguard is a custom-fit guard that cushions your teeth against clenching and grinding, which can wear down enamel and load the jaw. A retainer holds teeth in place after orthodontic treatment so they don't drift back. Both are made from a digital scan of your own teeth (no goopy impression trays) for a fit that's comfortable enough to actually wear.