Why use Snap and not one of the 19385791058 competing standards?

Why not?

Redundancy isn't a bad thing

It's certainly annoying. If a customer needs to have a few different platforms installed, they won't really learn to use any of them. If they have one or two, they'll likely get more value out of them.

If everything was available on every packaging platform, then I'd say the user choice is valuable. However, if most apps only support one or two platforms, it becomes fragmented and you lose most of the benefits of user choice. I like having lots of Linux distributions because most apps work on most distributions. I don't like the current situation with Snap/Flatpack/etc because that's not the case.

I don't use snap personally but surely valve of all companies would keep the packages up to date, meaning no difference no matter what you choose

Same reason they are using DEB in Ubuntu at the moment: it's the one that best integrates with the particular OS.

Supporting Snap isn't the same as supporting Ubuntu.