Bitcoin Forum

As far as I know, the incentives are.

For run a bitcoin node: one can send transactions without depend on third-parties and query the blockchain without revealing what is looking for.

For run a private electrum server: one can run a SPV wallet that store the keys in the client side, relying in the server to propagate transactions to the network in a trustless manner.  

For run a public electrum server: the same as private electrum server.

So what are the incentives for run a public electrum server? Help others offering the server for propagate transactions for free may not be an incentive at all.

You're essentially running your own private server and allowing it up for other people to use afaik.

You get the benefits of running your own private server (enhanced privacy of your funds and addresses and don't need to trust who you're connecting to) but you're also giving something back to the community for people unable to run their own or who don't have as readily available resources.

Realistically, improving bitcoins infrastructure also improves the chances of it being adopted by more people.

There's also donation links that users of electrum can access and I've seen some of the longer standing nodes receive some donations to them so that might be another incentive but there weren't as many electrum servers at the time - I think they were mostly the original devs.

For some, this may also count as an incentive:

Some want incentives and the only one I can know of is linking addresses together and maybe also to a list of IP. Maybe this information can be sold to chain analysis companies although I expect they have a fair share of SPV wallet servers themselves.

Some don't care about incentives and simply want to give back a little. Maybe had gains from Bitcoin, maybe they are happy to help Electrum network... In the same way many developers did work on tools without asking for incentives, others simply help the network without asking for incentives.