Are you saying that if you had a raw board from Home Depot, for use say as a shelf, with these other tools you wouldn't need sandpaper at all to make it smooth?
I have taken boards from the sawmill - nothing but saw marks - and made them table top smooth with plane and scraper. As someone mentioned, broken glass can be used, but I would suggest Kevlar gloves! Besides, you don't have to deal with dust and that can be very bad to inhale.
how do you keep these planes sharp? Can you take the blade out to sharpen it?
You have to learn to sharpen your tools.
A teacher is easiest, but there are books, magazine articles and CDs galore. I use 4 or 5 grades of oil stones and a 1900's razor hone to sharpen and maintain (hone) my edged tools. A plane iron is removable and needs sharpening right from the box and periodic honing as you use the plane. The sharper the better.
Your work is easier, the finish better and the tool safer.
Welcome to the journey!
Remember that the craft is long, life is short! You will never learn it all, so keep learning.
Knotty stuff might be used, but we aren't talking about gnarley pine 2x4s. We are talking about walnut, oak, mahogany, etc. A good pine with straight grain can be used and some slow=growth southern yellow pine can be harder than seasoned oak!
If what you want is shelving, use hardwood for strength - softwoods sag too soon
unless the span is short!