In a saddle stitched booklet the bulk of the paper causes the inner pages to extend or creep further out than the outer pages when folded. When trimmed the inner pages are narrower than the outer pages, counteracting the appearance of creep.
The illustration shows an exaggerated view of how the inner pages of a saddle stitched booklet creep out and extend beyond the edge of the outer pages when folded.
Creep varies depending on the thickness of the paper and the number of pages. Thicker paper and more pages will accentuate the creep. If there is no creep allowance, when pages are trimmed the outer margins become narrower toward the center of the booklet and there is the possibility that text or images may be cut off.