It's to ensure a consistent environment for all developers, with the resources required. E.g. they mention GPUs, for developers working with GPU-intensive workloads. You can ship all developers gaming laptops with 64GB RAM and proper GPUs, and have them fight the environment to get the correct libraries as you have in prod (even with containers that's not trivial), or you can ship them Macbook Airs and similar, and have them run consistent (the same) dev environments remotely (you can self-host gitpod, it's not only a cloud service, it's more the API/environment to get consistent remote dev enviornments).
Yeah, exactly. Containers locally are a basic foundation. But usually those containers or services need to talk to one another, they need some form of auth and credentials, they need some networking setup. There's a lot of configuration in all of that. The more devs swap projects or the more complex the thing you're working on the more the challenge grows. Automating depedencies, secret access, ensuring projects have the right memory, cpu, gpu etc. Also security - moving source code off your laptop and devices and standardizing your setups helps if you need to do a lot of audit and compliance as you can automate it.
