A lens cap protects the lens from dust, rain, scratches and bumps when you aren't looking through the scope. On a thermal scope that matters, because the lens is an expensive part to replace. Take it off when you set up and put it back on when you move.
No. Lens caps are sized to a specific scope body and lens. This one is made for the Pulsar Trail XQ50 and XP50, so it may not fit other Trail models or other brands. Check the model name on your scope before you order.