Keep seeing "do a factory rest" before installing a ROM. Why?

[northendtrooper]

The only reason I went factory reset is because I royally F*** up on my phone and started back to scratch. So I am having reservations of factory reset.
If I have to then I have a couple of questions.
-If I don't like the ROM, can I go back to one of my backups?
-Will I loose any data from my backup or Images from clockwork mod?
-Will I loose my root?
Obviously my biggest fear is loosing my chance to go back to a previous ROM. Let me know so I can possibly go head first into the new Jelly Bean.

 

[guidot]

What recovery are you using?

Generally speaking, everything will go swimmingly, especially once you have created a backup. If you don't like the JB rom you can go back to recovery and restore to the old system.

Always create a backup, then factory reset (which in stock recovery would wipe everything including pictures, but TWRP and CWMR exclude the /media folder from wipe), wipe dalvik, then wipe system. Then flash your new rom.

 

[guidot]

Also, if you are going from ICS to JB, you HAVE to do a wipe. There is no option. If you don't, it probably won't boot.

If you need to save other things like SMS/MMS and call history, there are apps that do that.

 

[northendtrooper]

Smashing. Thank you good sir.

 

[dezymond]

It's recommended to do a factory reset to make sure the ROM you're going to install will work without any major errors. You're starting the ROM off on a clean slate.

As guidot said, make backups, I cannot stress that enough. So many times I hear people flashing and then getting mad about losing info when they don't like the ROM because they failed to make a backup. What I practice is making a backup of every ROM I use as soon as I got everything set up the way I like it. I then plug it into my PC and then move the backup to a folder in my PC with a bunch of other backups. There's been a few times where I had to go to my PC to get one of my ROM backups and I was glad I put it there.