not just cheaper though

even subconsciously $15.55 will not be that better than $15.56

but in a change from $20 to $19.99 the whole first number is smaller, and that gives our ape brains the feeling that it's not as expensive

to reveal the vibes your brain operates on, think about bigger numbers. Imagine yourself to be in kind of a rush, you want to buy something, but family is waiting, or you need to walk your dog, or maybe you're doing shopping before work, regular life stuff,

first scenario

an identical item is sold for $2920 in the first store you visit, and for $2970 in the second store you visit. The stores are an inconvenient travel time away from each other. Do you go back to the first store?

second scenario

now, an identical item is sold for $2975 in the first store you visit, and for $3025 in the second store you visit. The stores are still an inconvenient travel time away from each other. Do you go back to the first store?

though the difference is still $50, the jump from $2975 to $3025 feels more significant than $2920 to $2970. And obviously many of us will go back to get the cheaper option in both cases, but there's a lot of people on this planet who have money to spare but not the time, and a lot of other circumstances too, marketing people know it and will do their damnest to sway you to buy their product