Does it actually reduce the crispness and clarity or brightness?
Yes, a screen guard, preferably a glass type one is a recommended addition to your lumia 640xl. Your screen, even though it's a Gorilla Glass 3 is still prone to early scratches and shatters when it falls on the wrong place. A glass screen protector will add another layer of protection to your phone so that it will catch all the screen impact when your phone is dropped. The protector will be the one that may shatter instead of the screen in case of an accident which is great so I recommend to buy atleast two pieces on eBay, or anywhere else, so that you have a spare one when it breaks. Only costs from 2 dollar to 10 dollars. I got a decent one on eBay for $7, which has 9H hardness, round edges, oil resistant, and scratch resistant so it is harder to break.
The screen will still provide the same colour and quality when using a glass screen protector. The plastic type ones may not get the same sharpness.