Glassine pages are the smooth, translucent, tactile, delicate pages that interleave every page of all our albums. Flipping them over is one of my very favourite ASMR moments as it makes the best ever, soft yet crisp sound.
The fundamental strengths of glassine paper is that it is air, water and grease resistant so excellent at protecting the photos from the elements and also your cup of tea and your sticky fingers. The paper sheets also protect the photos from each other - so when you close the album you don't have one photo ( with all its inky chemicals ) touching another one.
My favourite thing about them however is that they make the photo album somehow feel real. My wedding photo album looks and feels like my parents' did and it's the traditional element to it that I actually love the most!
So, the question is 'do you need them?' And my answer would be that it depends completely on how you're going to use your book. If you are going to use it mainly for photos then they will protect them over time and keep the colours vibrant and the marks/stains on them down to a minimum. However, if your book is predominantly for written messages eg. a book for a work colleagues to sign or for wedding guests to sign and photos aren't really involved then no!