Here's a good reason for root/ROMs.
My Asus Transformer TF101 tablet got updated to Android ICS with an official OTA update from Asus. Ever since, myself and many others have been experiencing a bug called SoD (sleep of death) and random reboots.
Mine is especially bad with SoD. Basically, whenever you let the tablet go into standby, when you come back and press the power button to wake it up, it doesn't. The display never turns back on. So you have to hold down the power button and reboot it. This happens to my tablet every time, within one minute of standby. So I'm forced to run an app called WakeLock that prevents the tablet from going into sleep mode. This greatly increases battery usage when the tablet is idle.
Asus has done basically nothing to fix this. The dev community has been trying all sorts of stuff, and solved it for many people. Unfortunately, all methods didn't help me.
So today I checked out XDA, and saw they have ROMs for the new android version Jelly Bean. They are very early and still have lots of bugs, but I just flashed one and my SoD issue appears to be gone. Its too early to tell for sure, but it's already a great improvement as I lasted over 30 minutes without an SoD, something my tablet has not done in months without wakelock.
Also who knows if the TF101 would ever even get JB officially from Asus.