Much more effective than a CPL
Polarising filters are known to modify and remove some reflections at certain angles but don’t work to fully block the glare (especially at night). A good quality CPL is expensive, they change the colours in your photo and you’ll need adapter rings to fit them to all your lenses.
Fits your lens
This means you’re free to move around, change your angle and adjust your camera settings without having to detach anything. Leaves no marks on the window. This’ll keep you on the good side of venue managers and their staff! The ULH is made of silicone and is designed to rest against the glass, rather than attach to it.
Can be used freehand or on a tripod
The freedom to move around without any complicated setup is a real bonus in popular tourist locations and zoos where lots of wandering people make available space limited. Not to mention many of these venues have banned tripods entirely.