Willem opened a Nikon lens to look at the inside, and found why quality lenses these days are weather sealed.
I am not a qualified lens technician, and please note, what I show here are lenses that I destroyed completely. I wanted to use their parts for something else, and opened them up just out of curiosity to see what they look like. Please note you should not do this at home, as you will destroy your lens. Lenses should be opened only at a service center.
This is how a 24-85mm G lens looks when you open it. Modern lenses are full of electronic magic just like our cameras. Therefore they should not be opened by people like me.
Here is an older lens, a 35-80 D lens just before I threw it in the dust bin, I also opened it to look. When I pulled the two body parts apart, I was greeted by a sight of dust and grime that collected in the lens over about fifteen years of use. The scary part is that the opening where the two body parts get together, was sealed with tape by the manufacturer, and still all that junk was able to work their way past the seal and enter the lens.
This is just a small lesson why the more expensive lenses are weather sealed these days with proper rubber seals and other means. Just imagine how you will battle to get the lens clean if that dirt make their way to the lens optics. Now please take care of your lenses – my lenses were broken beyond repair, and I broke them even further by opening them. Take care and rather buy weather sealed lenses for better peace of mind.