My wife asked me why custom plates were so important to me - since she doesn't particularly like spending money on things she considers luxuries (don't ask what I had to go through to get the okay to buy the F30).
I told her that I have had custom plates on every single car I have owned except my first. Nothing ostentatious, usually just something that refers to the make and model of the car - something easy to remember. In my line of work I find myself signing in on contractor sheets at a variety of sites and most want your rego plate so that they know who to contact if they need your vehicle moved. Having something easy to remember streamlines the process for me.
It's function over vanity in my case, but I don't begrudge anyone the choice to make their proverbial mark.
2016 F15 X5 XDrive30d M Sport
2012 F30 320d Luxury