It seems like Pure can be rooted but no one reports putting a new ROM on it yet ... I think. What is the benefit of rooting it in this case? I'm pretty new to this ... is it to have more de-bloating options?
Rooting allows you to do pretty much whatever you want to it. Stock commercial phones are limited in their functionality and with regards to access between the apps themselves, and between apps and hardware.
I own several different old phones. Depending on the capabilities of hardware, I have set some up as security cameras, daisychained a few so I could reach wifi while in a deadzone. Others I've used to experiment with in order to learn coding, programming, and app building. Mobile forensics it's a big one, since you can't create a forensically sound full disk image of a phone without it being rooted. Of the ones that I own that have no root strategy, I'll still try and get root access by running exploits and testing ideas people have on GitHub. It's fun.
I recently installed Metasploit Framework on nearly all of my phones, rooted and unrooted, though with the rooted ones I also have various penetration testing apps and command line programs. Again, it depends on the hardware.
My best phone was actually a Moto G7 Play that I was able to fully install Kali Nethunter on, and turn it into an HID keyboard in order to bruteforce lock screen pins. Also used it for MITM setups.