I just did a clean install on my late 2013 27" iMac. What I did was
was back up "data" in my home directory. Although that was just
really an additional precaution as the majority of the data was backed
up by a variety of methods (live by the concept, "one backup is no backup"
since the backups can fail or get corrupted). Also I had a clone
of my installation as another precaution.
I had created a USB installer and wiped the drive, installed Mojave, installed
apps that I use most frequently and then began copying my data.
So, did I have to do it? Probably not . But I had been doing in place upgrades
since Mavericks and the iMac, when new, had stuff migrated from a Mac that
had in place upgrades from Leopard.
So, I just decided that it was time for a fresh start. Even though I do try to keep
things "tidy", I am sure over the years I have been collecting a lot of flotsam and
jetsam and just wanted to eliminate any "ticking time bombs".