Tile and grout are not waterproof. Water passes through grout lines every time you shower, and what happens next depends entirely on what's behind the tile. A properly built shower has a continuous waterproofing membrane on the walls and floor that sends that water to the drain.
The shower floor needs a consistent slope toward the drain so water never sits. Getting that slope right before the membrane and tile go down is one of the most important steps in the entire bathroom.
Large-format porcelain looks clean and modern, but it demands a flat surface. We use leveling systems on big tiles so edges sit flush and you don't feel lips underfoot. Niches and benches get waterproofed and sloped just like the rest of the shower.
When you're comparing bids for a shower remodel, ask how the shower will be waterproofed and how the floor will be sloped. If the answer is vague, the shower probably will be too.