Good tip- In theory,the main value is that the police department will have the description of your bike, including serial number, in the event it gets stolen. They can't enter it in any crime database without the serial number. Most folk have no clue where they have the serial number written down. EPD has recovered numerous high end stolen bikes in Evanston over the years (several come to mind from the west and east coasts of the USA) where the offender was the second thief, and had just re-liberated it from someone's garage or basement locker. Plenty of locally stolen bikes were also recovered. In theory, even if the registration sticker gets removed, they can do a name search for your name and pull up the serial number.
So what's wrong with this picture?
The city used to charge 50 cents for this, and required the bike be brought to record the bike's physical details (which included running the serial number).
You get what you pay for.
Now, according to the link Joanne provided, it's free, is all on-line, no one looks at the bike, and apparently anyone can register any bike, whether it's theirs or not, and they'll mail you the sticker. If you can detect any sense in this new procedure, do let me know.