Why does the surface matter more with big tile?
Large tile needs a flatter wall or floor because the tile edge has less room to follow dips, humps and bowed framing.
If a floor rolls or a wall waves, the edge of a big porcelain tile can catch light and create lippage. Renan checks the surface first, then decides whether the fix is floor prep, subfloor work or self-leveling compound.
That prep is not decoration. It is what helps the finished surface feel flatter underfoot and look straighter across long grout joints.

