I got 1TB and I'm very happy. 256GB is just far too small for my uses. I have two DAWs and lots of sample libraries installed and developer tools for working with large codebases. I've had my machine for a month and already have only 512GB free. The internal storage is fast and there's nothing extra to carry around too.
If you're just doing web browsing and light office work then given how much lives in the cloud these days 256GB could be fine for a lot of people. It's a good rule of thumb however that you'll always want more storage than you think in the end.
One other reason would be longevity. I have a Late 2009 iMac that I still use from time to time. I also have a 2008 Dell XPS Studio that I use as a backup system. More storage gives you flexibility as you could use it as a NAS in the future or repurpose it for work which requires a lot of space.
I think that you can have a mix of storage levels for systems. My direction on mobile devices is less storage in the future as I'm doing more in the cloud. But there are some cases where I want a lot of storage, typically to serve other systems. My solution is to do that with a Windows system as I can easily add storage cheaply.