Remember, if you got an item for $70 instead of the $100 regular price, you didn’t save $30, you spent $70. That’s what I tell myself when I see discounts on stuff that aren’t immediate needs.
I take your point, and you're absolutely correct in that if you'd have never spent that $100 on whatever whizzy gadget you're looking at then you didn't save a penny no matter how good the sale is.
However, life is more than mere survival. If I were to buy something that I was going to get anyway then that's money saved even if it's not an immediate need. The caveat being that I hate "lightning deals" and other short time sales (damn you Musician's Friend and your Stupid Deal of the Day) because in the past I spent a lot of money that I didn't have trying to get a deal on something that I didn't take the time to properly research and wouldn't ever use.
Furthering the idea that 'life is more than mere survival', I want to add that choosing to buy or not buy an item affects more than just you.
When you buy a $70 item from Amazon on sale, yes, you've spent $70 and not 'saved' anything at all. But even if it was a good price, you should consider the added cost is that you gave your $70 to the evil empire. Amazon got your money, and you got your trinket. Surely it would be better if you spent the $70 somewhere else. Give it to someone that you like more than Jeff Bezos. I'm sure you can think of someone like that. You probably won't be able to buy that exact same trinket for $70, but surely you'll get something else you want - and you can actually help someone else at the same time rather than entrenching the power of exploitation.
And sometimes you can choose not to buy something that really will improve your life until you see it on sale. Maybe you don't need it, but it's more affordable now, so you go ahead and buy it and it makes your life better.