I've personally wanted to reference an ad after purchasing something or in other situations and it's gone. Again, think of physically clipping a newspaper ad. Pretty much just like http://wayback.archive.org/but on a more granular/niche scale.
Reasons I personally like to revisit old ads sometimes:
- Seeing prices that people asked for an item at different times, and the full text of the ad and pictures so I can compare extras included and condition of item
- Something I bought and I don’t remember how much I originally paid for it
- Things that I did not buy, but I want to remember for the future and perhaps buy similar item from someone else in the future
- As a substitute for buying the thing. Instead of buying an item now I can keep a copy of the ad and over time I can think about whether I really need that thing, or if it is not as necessary as I impulsively had imagined
- etc