THEY ARE!!!! It's right in the name! Like @sleepnoises said a while ago, they're for the quick putdowning of ideas! to catch the stuff that comes out of your brain!
(I don't mean to say that pretty sketchbook pages are bad, but it's important for your health to do quick scribbly stuff too. if not in a book then perhaps on small scraps of paper)
The dinosaur is very glad to see so many doodles in the notes!
Also lots of comments with very good suggestions for people who have a hard time starting, like starting on the second page, just signing & dating it, numbering the pages, or using the first page to put samples of all your usual pencils and pens and such.
Here's how I started my new sketchbook, which I got for 5 dollars at Dollarama.
I would have drawn on that first blank page instead of just signing it, but it was partially glued to the endpaper and wouldn't lie flat.
(Also, since the sketches are about making velvet mites with little wired legs, I must mention that if/when I get around to doing that it'll be posted on @vincentbriggs where all my non-dinosaur arts go.)