Supporting security towards the network is one of the major drivers (in my opinion). Then immediately after that, the fact that if you are distant from everywhere, you also want your local node to get access to super speedy transactions... in case you are also building more infrastructure towards side-chains.
Then, after that, there are tons of applications... from supporting the community with a public node (VERY EXPENSIVE, and props to anyone supporting it) and providing the global infrastructure with an added value towards decentralization, to supporting specific apps, ventures etc... there are lots of options one could support nowadays with software.
Are you interested in starting a node? If you are comfortable with coding, I would advise giving it a read on HIVE side-chains and then because you will get hooked, running a HIVE node too. 😋
A Witness node won't help with local access, for that you would need a full node.
Depends on the plugins... I agree,
For HE it's enough everything without account history for example. DLUX types and sorts, it's different, but can also be segregated with the only thing that needs history.
In essence mostly for get blocks, hive state, broadcasts and if one will use wallet functions.
But
Another good point, can you link to the further reading on Hive side chains?
For Hive Engine, https://peakd.com/engine/@rishi556/how-to-set-up-a-hive-engine-witness-step-by-step-guide
For DLUX type side-chains... https://github.com/dluxio/dlux_open_token
Note: there are more than DLUX but based on similar tech...