The main reason why you would want to downgrade is because you want to run homebrew applications, games, emulators, etc. This may not be very easy to a beginner but I found a link that will certainly answer you questions. You can find it here.
When you downgraded to 1.5 you lost some funcionality like the web browser, the ability to play some new games, and other stuff. Fortunately, there is a solution for this. It's called devhook. What it does is it emulates a firmware you want from the memory stick, giving you all the functionality of that firmware. Right now, the highest firmware you can emulate is 2.71 which is enough for almost every game out there. You can find more information about devhook and its installation right here.
- Custom firmware
With 1.5 you can also use custom firmwares. These custom firmwares add some very nice features to your psp. I'm currently using harleyg's custom firmware which you can find here. The zip has instructions to install it.
Thanks and keep visiting.