The retailer with the loudest sale marketing isn't always the one with the best actual price - comparing across a few options before buying, even during a major sale event, regularly turns up a better deal elsewhere.
A retailer's own sale event competes with everyday pricing at other stores that aren't running a special promotion at all - checking a couple of alternatives takes a few minutes and sometimes reveals a lower baseline price elsewhere.
A slightly lower price that comes with a shipping fee or a restrictive return policy can end up costing more overall than a marginally higher price with free shipping and easy returns - the total cost matters more than the sticker price alone.
For anything expensive enough to be worth the few minutes it takes, a price comparison tool checking multiple retailers at once removes most of the manual work involved in confirming you're getting the best available price.