To me, the iMac G4 is a genuine productivity tool that I seriously and regularly use.
For comfort, get an SSD and some RAM. Although, so far I have only put an SSD into mine but no more RAM yet. I have 1GB.
It's ironic that my iMac G4 boots many times faster than my newer main computer ever since it was updated to one of these new macOSes that change the hard disk format......
I go to the iMac G4 room several times a week, and spend hours with it. It's a perfect distraction-free environment especially for doing any sort of writing, for example responding to e-mails, doing work related writing, or brainstorming. It's distraction-free because of the computer's limited capabilities, and because on it, you won't get the urge to multitask; Procrastinating by browsing stupid websites is my worst enemy, but on an old machine you just can't do it so you focus on the task at hand instead. That machine gives me an environment that modern computers can not. Especially when I'm working on some complex project, it's very important for me that I don't do all the brainstorming sitting in one spot with the old ideas, but I can get some fresh perspective by writing on a different machine in a different space occasionally; You think different when you do that! It's a bit similar to the way how, if a musician composes using a guitar, the compositions they come up with are different compared to compositions made on a piano. This variety does help.
With everything, a lot of the software at the time was much simpler, which means less features and less clutter. I feel like it's a great environment for whipping up a wireframe or draft version of pretty much anything, be it text, visuals or audio; You just get to the point and don't overthink things because the software only has the necessities and the computer just can't handle bloating everything to extremes.
I have a small USB memory stick in my internet router, via which I transfer the draft (and finished) works from iMac G4 back to my main computer, where I either archive it somewhere, send the content to someone, or finish it by adding some final touches. For example, because Logic opens GarageBand files (but not vice versa), it's possible to draft some music in GB and then do the actual heavy lifting in Logic on a newer machine. I also have installed the very same Photoshop version to both machines, literally from the very same DVD.
Also, I'm always listening to music on the iMac when I write; I just found the Audion player with nostalgic skinning features, and while I don't find its playlist features intuitive to use at all, it's a great little player to play internet radio streams in.
iTunes seems to be able to download podcasts and update them to my iPod Nano 4, which is nice. There's something wrong with some podcasts though, iTunes gives an error and refuses to download new episodes so I'll still have to update those from my main computer, but most of the podcasts I listen to do get delivered to the iMac and this makes updating the iPod easier because the dock that it sits in is in the same room next to the iMac.
My main computer has a CD drive so I don't have the issue of not having one at hand, but many people with modern Macs today do lack a CD drive, and with the iMac G4 you can rummage through your abandoned software CDs, listen to and rip music CDs, or watch DVDs. It plays back video files from Finder horribly poorly, but DVD playback is all smooth. CDs are a bit noisy though, so I mostly listen to them with headphones on.
Because of the great customizability of the user interface such as icons and all that, an older Mac can give you a very personal, homely looking space that's also sadly been removed from current Macs. Sites like Iconarchive or Deviantart still have huge collections of downloads. The latter also has some docks, and Candybar is the (now free) app that allows you to do all the customization.
I've been able to connect a modern (unsupported) MIDI keyboard to the iMac, and it works as a basic keyboard (excluding the keyboard's special controls that are about controlling the software itself, in this case GarageBand).
I can use Applescripts to command the Philips Hue smart light system, works from the G4 just as well as from my main computer, no trouble at all. For example, I might run an Applescript from the iMac which checks what time it is now, sets the lighting in that room just perfect and appropriate for that time of year and day, and then it switches off the lights in the other rooms where lights aren't needed.
My main computer is in the living room and already with so much peripherals around it, I didn't want to have the printer there too, because it's an eyesore. So now I also go to the iMac to print because it happens to be in a place where the printer can be nicely hidden.
So I do go between the two machines quite a bit or exchange files between them via the USB stick in the router, but I don't mind this at all because the sheer productivity and fast boot time of the iMac make up for it.