An original UZO still in good condition:
The front lenses are considerably larger than those used by the crew. The lenses being larger was as important as its magnification/precision. The larger the lens is lets in more light so you can see better at night compared to a regular set of bincoularsl. Zeiss made high quality optics to begin with, any UZO was special in that they were designed specifically for the u-boat per the above reasons. The UZO would have been the most precise optics (excluding periscopes) set on the boat, and were only taken out when for use when attacking.
Edit: From the sound of thins it doesn't seem like you've seen Das Boot. If that is indeed the case you should get it as soon as you can get your hands one (rental, Amazon or whatever) and watch it immediately. It's the most realistic and historically correct u-boat movie ever made, you can learn a lot from watching it if it's your first time.