There is the Windows Store, but with its limited OS support I'm not sure how successful it will be, but I can see the need for it. In fact, why doesn't Steam itself extend its service to include software other than games? The only downside I see is that software developers may be reluctant to publish there since it would add a requirement to their software that Steam be installed on the user's PC. For software developers, the less the requirements, the wider the audience base.

What do you think?

In fact, why doesn't Steam itself extend its service to include software other than games?

Why do you think such a platform is needed? Just to aid in discovery? To help with updating? Personally I'm happy finding and downloading software the traditional way.

For the end user:

1. Guarantee that what you see is what you get - no hidden adware or spyware. (Clear license). Or, more practically, at least knowing that the software in there has passed through whatever quality control requirements and measurements the platform enforces.
2. Actual user reviews.
3. Easy updating.
4. No funky DRM - you can use your software on all your computers.
5. Easy navigation - finding the best (=most well-reviewed) software for a particular task becomes a few-clicks process.
6. Discovery - being informed about cool new software.

A lot of these can be had individually by other means (for examples, "discovery" can be satisfied by visiting some tech-focused sites like lifehacker), but not together in one place.

The platform would be very helpful for the independent software developer/publisher. The reasons are the same that people publish games on Steam. Discovery is the main one.

In fact, why doesn't Steam itself extend its service to include software other than games?

Steam Library: Software :)
The overwhelming majority of software on there still seems game related in some way -- there are a lot of engines and content creation packages -- but there are a few other things in there too.
Why do you think such a platform is needed? Just to aid in discovery? To help with updating? Personally I'm happy finding and downloading software the traditional way.

Most platforms already have such a store. iOS and Android each have their own stores, obviously. Mac and Windows each have their own, plus Steam, Origin, and bunch of other game-specific platforms.

The only odd one out is Linux, although I don't know very many Linux folks who like the idea of an AppStore in the first place...

+1

Linux's store is apt-get.

+!

Doesn't Ubuntus Unity interface have something similar to an app store?

I think the OPs main question is regarding Windows though. On the other platforms like OSX the app store has been welcomed by users and developers and there is a lot of good stuff. On windows it seems like people go out of their way to avoid the store.

I use the Windows store, the problem is that most stuff you can buy there are Modern UI style apps (as in Windows 8), and you kindof need a Microsoft account login on your system (not strictly required, but really awkward if you don't).

"There is the Windows Store, but with its limited OS support I'm not sure how successful it will be" - this is a curious comment. How is the OS support limited? It's the Windows store, so it supports .... ahm Windows.