Many RV parks and boondocking spots you visit won't be very level. You'll want your RV level not only for comfort reasons, but also to ensure your fridge works properly. Leveling blocks are the easiest way to get your rig level.
The cheapest option is to just use some stacked 2x6's to level. The reasons for using plastic are 1) it's not going to rot and 2) they're usually specialized to be easier to use (either with lego-like stacking, or curved levelers)
Flat leveling blocks look like legos that you stack together and drive over. Curved levelers are a rounded piece of plastic you drive on and insert a chock in the gap it creates.