There is an option to not erase the installation destination for Ubuntu, but it's probably not what you want.
First, Ubuntu has to be installed to a compatible file system, which your system probably doesn't have if you haven't already been using some other Linux distribution. In that case, you need to format (erase) a partition.
Also, even if you somehow had a compatible file system, the Ubuntu installer would still overwrite the space where it needs to install the system.
What you're currently describing as a "backup" is not really an acceptable backup. You need to have some other place to back up your important data. Having data duplicated on a different partition on the same drive doesn't do hardly anything to protect you from the most common causes of data loss.
That's up to you how and where you choose to do backups. We can't tell you what you need to back up and the best way to do it. You need to determine that for yourself based on your own threat model and the importance you put on your data. But you should be aware that if you don't back up your important files, or if you don't put them in a safe or redundant space, it's only a matter of when not if you're going to lose it.
You're playing with fire to not have good backups in an alternate location before installing operating systems. What if you mess up?
