I found this analogy to be simple:

Your 1GB hard drive is almost full, you have 2 options:

1. Buy a 2GB hard drive to replace the old hard drive
2. Buy another 1GB hard drive, and a RAID controller and set up a Raid 0 array, so that the Raid software will move 40% of the data from your old hard drive to new drive , so it becomes not full again. Then you have total 2GB of space in your new array, which functionality wise equal to a 2GB hard drive

I think any normal people will just go for the first option, only geeks and technical interested guy will try the second approach, and eventually many of them will give up on the second setup because it is just too complex to implement and maintain, and a Raid 0 will have higher risk of failure, it does not worth the effort