Use blocks to clip through walls. If you stand just off a wall - as in, not directly touching a wall, but a tiny amount away from a wall, and place a block at your feet, you will clip through the wall.

Could probably be useful if done in 000 or 001 to get into the rooms with the trees and bypass those puzzles. Maybe 053 to skip the blue side and go directly into the green tree room? Other than those, can't think of too much in the midgame since you're usually carrying cubes that you need, so the doors have to be open. Possibly in the 800s if you're able to clip past those doors.

Can use this to skip to the endgame:

Yeah, I saw that and have added a wall clip subcategory.