All it takes is for the initial notice to go missing in the post (a look through here and PePiPoo will vouch for how often that can happen) and then you end up dragged into going to court over what would have been a £2 pay and display ticket.
When the Royal Mail ran their own test a few years ago (and these would definitely be clearly and perfectly addressed) 17 out of 15,000 were lost (that seems extraordinarily high to me).
Dreadful problems can result (though the chances of a reminder also getting lost are down to 1/million approx).
Given so much is now on line the vast majority of post now is actually important.
There seems to me to be a particular problem with post from the DWP, HMRC (though most is online) and individual DVLA letters. Often those that I receive are not properly visible in the window envelope. The service the NHS uses is also poor in this respect.
Government departments seem to be putting in place systems to help. Everytine (as far as I can tell) DWP send me something they also text.