Well, the idea here is to confirm to yourself that when it is factory fresh (without your backup on there) ... that it is operating properly. So, that means setting it up as new (but you want to keep your backup in reserve for later restoral, just not now). And when you start it up, you need to see how it operates. "IF" problems show up at that stage (and without your backup on there), you can show it to Apple and get resolution of those problems, and show to Apple that it doesn't have to do with your backup data, but it's solely doing that in a factory fresh condition. Now, many people have not had problems at that point, and I'm hoping you don't either ... and then they just go and do the backup restoral, since they've confirmed everything is working okay at that point.
Now, going back to when it is in factory default condition ... there have been those users (who have reported on this forum) that they've taken their machines down there in that factory fresh condition and Apple has seen the problems and has replaced the iPad for them. Now to me, that means it was a hardware issue, because Apple does not replace machines for "software issues" ... they just fix the software issues. So, the fact that they had their iPads replaced means that there was some kind of hardware issue. They got new iPads as a result.
But, let's say it works for you, so you don't get a new iPad ... :-) ... then you restore your backup after that. And I'll tell you that some people have found out when they restore their backup, the problems return. And that means the "trouble" is in your backup! Just to give you "one example" of the trouble that could be in your backup ... one user had Apple do a special diagnostic on their iPad that was giving them trouble. Apple found one app that was causing them trouble, and they said to get rid of it, since the developer had not updated it for iOS 8. He did and the trouble went away - just from that one app. So, an app you have on your iPad could be your trouble, too. But, I can't predict that ahead of time, so I can't tell you that it's going to be your problem. Others have done the reset to factory default condition and restored their backup and that's it ... they're all done and everything is working fine.
I just can't say ahead of time, what is going to be your problem ... a replacement of your iPad because of hardware issues, or a clean and fresh system install to get rid of corrupt files, or an app that is bad and you need to get rid of that. It's just that this is a "methodology" to troubleshoot your iPad and in doing this, you'll arrive ... eventually ... at what your problem is. I hope that explains it ... :-) ...