When baths were solid, bath trim sets made of ceramic, and were used to hide the gap between the bath and wall. They had internal mitres and coved ends. The top and bottom edges were siliconed for extra waterproofing.
Then they came up with plastic bath seal which was fixed with adhesive to the wall and sat on the bath ( which often moved) and this had a flexible edge seal. These often kicked out the first row of tiles. However over time the baths moved considerably and moisture would get under. The only way to remove this bath sealer strip was to take the first row of tiles off.
A bath today should have a wall mounted baton to take the bath frame, legs correctly fixed to a stable floor, and a good bead of Silicon before and after tiling.
Job done!