One instance of spoiler gradient use is at the end of a sequence to spoil Mxy prior to the next RF excitation pulse.
Another instance is either side of the slice select gradient which is applied during the application of the 180° refocusing pulse in a SE pulse sequence. They are applied because the 180° pulse may not be exactly 180°, and it may not refocus everything at time TE as we expect. The crusher gradient after the 180° pulse destroys the FID (free induction decay: the decaying signal), the one before the 180° pulse is to balance the one after. Zipper type artefacts may be seen in the absence of spoiler gradients, because the RF pulses used are not perfect.