The main purpose is bug fixing. Many applications work faster. Here is the list of changes:

• Drag an app icon across multiple pages in one motion, rather than having to drag it, drop it, pick it up again and drag it over to next page, and repeat.
• Contacts are now more responsive.
• Backup is much faster.
• Apple may have changed the calibration of the iPhone's reception "bars" while connected to a 3G network to reflect a stronger signal than before.
• Keypad loads quicker.
• Screen rotation in Safari appears to be smoother.
• Some apps may ask to be updated after the firmware upgrade.
• SMS typing faster.
• Google Apps works faster.
• Netshare still works (for those who use it).
• Works with iPod.
• Multimedia content is kept after update.
• No Support from Pwnage and WinPwn. If you update you'll loose unlock and jailbreak.