Lease is if you're not sure you'll keep the car, and you know/feel it will have good resale value after lease end. There also a possibility of paying less than car's 3-year worth. Here's an example of that:
I lease a car that costs $20000. Its a new car, and no one knows it's worth after 3 years, so dealer/manufacturer guesses the worth of the car 3 years from now (my lease is for 3 years) -- say, $9500. So, he says 'your residual is $9500, you get to finance $10500 over 3 years'.
'Okay' I say.
3 years passes, and I about to return the car. I check the market price, and find out that my 3-year old car is worth $8000 on the market, according to the Blue Book (NADA). So, I return the car after my lease ends.
If I bought the car, I would've paid $20000, then sold it for $8000. Total expenses: $12000.
Since I leased the car, my total expenses were: $10500.
This example was simplistic, but illustrates the point. Cheers.