Although the generator and the grid are now operating at the same frequency, they are not necessarily phase angles were to be connected to each other, a fault similar to a short circuit would occur and most likely destroy the generator and damage the grid.
Although the generator and the grid are now operating at the same frequency, they are not necessarily at the same position in the rotational cycle as each other. If two electrical networks operating at two different phase angles were to be connected to each other, a fault similar to a short circuit would occur and most likely destroy the generator and damage the grid.