Contact lenses change the shape of your cornea even in the long term. Under certain circumstances, it can take weeks until the natural shape of the cornea establishes itself after removing the contact lens. If you did not remove the contact lens sufficiently long ago before the initial examination, this could result in planning of the surgical procedure not matching your actual eye conditions and thus being poor. The consequence would be a disappointing result with mostly poor vision after the procedure.
You should not wear soft contact lenses for two weeks before the preliminary examination. Soft toric and hard contact lenses should not be worn for two weeks or preferably three weeks before the initial examination, and you should also take a break from wearing contact lenses before the operation itself.