Well, actually, now that I've been a mama for ten years, I DO buy gifts in advance. The reason I'm mentioning the mama part is because this is mainly a thing I do in the area of children's gifts. I buy my own dc's gifts for holidays, birthdays, etc. whenever the opportunity arises for a good deal/rare find/ etc. This helps break up the cost of holidays so we're not spending a lot of $ all at one time.
Also, when it comes to the verymany children's birthday parties my dc have been invited to over the years, as well as the verymany they're still yet to be invited to in the future, I find this way of gift-buying really
pays off. I used to go out and buy individual gifts for each birthday party we were invited to. But I found that, not only was I spending much more money doing it that way, but more time as well, because I had to make a trip to the store for just one gift on any given week.
So I'll be in a close-out type of store and see something very cool
like sturdy metal cash boxes that come in different colors and with their own set of keys for just a few dollars each because they're 'overstock' from another store (or whatever) and I'll snatch up a few -- kids love these things as gifts. Just one example.
Really I think this way of doing things just takes the imagination/visualization necessary when you happen upon something -- you would need to be able to see it's potential. I've trained myself to do this, now. So when I'm out and about it's easier to quickly identify items that would be good to buy for this purpose.