What it was to prevent was lifetime free warranty and upgrades. Buy a laptop, use it for 3 years, come back and say "I don't like it anymore.", get your money back, buy new laptop with upgraded specs... repeat.
And before you think this is theoretical, when I worked there we had a guy return a TV. It was a ~7 year old rear-projection TV that was bought before the 90 day policy was implemented. It had died well out of warranty.
So he returned it, got his ~$4k back, walked out on the floor and grabbed a $2k LCD and went home with a brand new LCD TV and $2k in his pocket in exchange for his 7 year old broken TV.