Oh I agree with you both on preferring to buy what I plan on getting on sale. Its the mixing of the two strategies for purchasing that I don't get. I am either going to save up and start looking at prices after I have saved up OR I am going to keep an eye on prices and buy the best deal.
If something is on sale then naturally it requires less money than the sum you're trying to save up. If you're saving for a $400 item but you think it might go on sale for $200, you can start looking when you have $200 saved up, no?
Or you might put an alert on ebay or craigslist right at the beginning because sometimes you can get stuff crazy cheap used.
So this mixing strategy might be a result of knowing that prices on a lot of consumer goods are arbitrary and variable. Plus it puts a timeframe on how long you'll spend looking for a good deal. Maybe people know that once they save up the full price, they'll be impatient to pull the trigger. After all, should they wait a month to see if it goes on sale? Three months? Hard to say. Whereas if you need 6 months to save up, you know you'll just watch the sales for about that amount of time. And if you've seen it drop to half price, even if you couldn't afford it then, you at least know that it's reasonable for it to go on sale for half price, which might give you the motivation to wait for that to happen again.
Of course, I agree with the caveats about people using "it's on sale" as a crutch or the fact that constant reminders of sales might reinforce in people's minds that they want something they might very well have forgotten about otherwise.