The main reason is to maximise water flow away from the surface as soon as possible to maximise drying time. One bad part of damp is it eventually gets behind the tiling system, and yes, inside the waterproofing. The biscuit (clay part of the tile) is only glazed (waterproof) on the front, so your tiles will start to dampen and soften from behind. The damp is now trapped between two waterproof surfaces, the waterproofing and the glaze. There are threads about how discoloured damp tiles look, apart from the obvious structural issues.
Then, with this damp, keep a look out for:
Even if you kill the surface mould, it may continue growing behind the tiling system.
If you use mould-resistant silicone in the gap, you can reduce this somewhat, but unless you over silicone to create a run-off effect, water will still pool, and enter tiles via the cuts and biscuit, unless you silicone over the sharp cut ends.
As for cutting tiles, it is no different snapping on an angle than snapping straight.
As for looks, mould looks ugly.
This process may take months, years, or never happen at all depending on water coverage and airflow etc.
But at the end of the day, I tell everyone the same thing. You gotta live there, so as long as you’re happy.