Open source developers want to genuinely help people instead of make loads of money by selling their data and products, that can be a factor, but it's also to help people understand the software and help you trust them.
With open source software you will get more in-depth help and more ?helpful help?. Basically you will get good community support instead of paying someone to help you.
By creating OSS you will get more recognised as a developer and your product might be more successful because of that, you will also reach more people trying to help you and find bugs and performance hogs so people who use your software will get a better experience.
If you make open source software people might also want to help you, though don't expect free work just because it's open source :)
If your project is popular on something like github you will probably get more users and so people trying to help you and search for bugs, etc.
Developers who develop OSS and when it becomes huge will feel more motivated to work and will mean more to them as people have either helped with code or they have complimented it or something similar.
Open source software usually cannot have messy code, well it can, but it gets quickly fixed, as everyone sees your code they will fix it and improve your code and when people don't see your code developers can be a bit more lazy and write inefficient and ugly code 'cause who's going to see it? Also bugs are less common in OSS as it has multiple developers looking at it a second trying to find bugs, well if they so choose to.
As the project is open source the developer can't put any backdoors and data collection things unnoticed. Also if the application is storing anything for example login credentials chances are that the security is better as people are trying their best to harden it, though there are some bad people out there trying to introduce backdoors, though it's rare and as it's open source it gets patched pretty quickly one time it happened with PHP and it got reverted quick.
If the closed source software you're using is lacking some feature or has a breaking feature, you cannot modify it, when with open source software you can.
Your open source project will be more available to more devices and architectures because people can modify the source and if your designed your project well - it might just work with only compiling on the specific platform.
For example if science kept everything secret we wouldn't know as much as we know, people who discovered antibiotics could have kept it secret until they died or they did and could have released it and made it available to the whole science so they could build, improve and help with/upon it. And the same applies with IT, IT develops faster when everything is easily modifiable and accessible.
People might try to improve your product, but they might break it, so if you don't make it clear in the rules of opening pull requests, they might get rejected.
It's a bad idea to open source something if you're the first one to create it, but it's also good because you might teach people something and they will value your product more than the competitor's copy