I've got an old Lenovo Tab3 850M table, running Android 6.0.1
It's pretty vanilla and I haven't installed any custom ROMs or rooted the device.
The battery is in pretty good condition and the table can idle for around a week I think.
It has an 8 inch IPS display, Quad-Core 1.0GHz ARM Cortex-A53 CPU, ARM Mali-T720 MP2, 500 MHz, 4-Cores GPU, 2GB of RAM, and 16GB internal storage.
The cameras are pretty terrible (even for 2016 standards), and the battery is 4300mAh.
First, I'd suggest flashing a community system ROM. OEM systems are both very bloated, and stop being supported much earlier than community ones, so they're not ideal unless you need to use it with some banking app or the sort. And if you don't need Google's services (de-Google, anyone?), I strongly recommend going for a vanilla system instead of a Gapps one.
Now, as stated in MargotRobbie's comment, one good use is as a media player. If you can sideload stuff like VLC and Librera Reader, you should be covered.
You can also use it for some lighter gaming, if that's your thing, as there's plenty of emulators, wrappers and engine implementations for Android.
You can also use it for running servers, if you do this sort of stuff.
And if you like to test around with softwares, a spare Android device is pretty good to have.
I checked the XDA forum a bit, I found this but I'm not sure on how to unlock the bootloader, install TWRP and flash the ROM?
I had once bricked this tablet while playing around with rooting it and I had to take it to a repair shop. So I'd appreciate if there were working unbrick instructions and also working instructions on how to install TWRP.
You also need to find instructions specific to your device on how to unlock the bootloader, and a TWRP specific to your device. From there, you do a clean flash through TWRP (erasing certain partitions and then installing the new ROM), and then, you should be good to go.
Eh on a tablet display its not like 1280x800 is bad for video. Unless you're a pixel peeper and you hold the display <6 inches from your face the quality difference will be negligible
There are people still looking for older cheaper devices cause they can't afford newer devices. If you are not using your device, imho it would be used better with someone else.