I don't get it, I may be missing some context thought: why the author didn't go for running said webapps directly on the VPS in the first place? Why bother running it on the Pi and set up the tunnel (which is a nice hack in itself, I get that)?
1. I have much better hardware in my house than any VPS can offer at a competitive price. This is particularly important for storage. Object storage like S3 is relatively cheap, but I just want a file system I can slap a static server (or whatever else I want) in front of. If you go for full block storage the price is way higher, and performance still isn't as good as a local SSD.
2. I hope the future of the internet involves more people self hosting over IPv6. I see tunneling solutions as an intermediate step to that world. If we make it easy for people to self host, it might even be able to help drive IPv6 adoption.
I've used similar techniques for doing a wide variety of operational tasks, so it can be worth it to take the time to figure out how to easily set up tunnels and reverse proxies.
Using Linode as a benchmark, a 512 MB VPS is $5/mo but an 8 GB VPS is $40/mo. But an 8 GB Rasberry Pi 4 is well under $100 with peripherals so after three months it will have paid for itself.
This of course assumes:
1. that the cost of powering the Pi is negligible 2. your home internet connection is stable enough and fast enough 3. your apps run as comfortably on the Pi as on VPS
A VPN is often just too complex to set up, and also requires some hardware in your local network; though often your router will have that already. Off course a Rpi can act as VPN gateway/server.
But solving it by flipping the "servers" around and running the "appserver" in your network then tunneling it to remote seems a really neat hack to avoid going into VPN hell.