There are cases where the service is not substituting software. E.g., something like Stripe or Twilio... versus software which is offered as a service because it's a nice form of DRM.

It is implicit, the point is this makes it explicit.

But why? It's obvious. Making it explicit has no benefit, and makes you waste a bunch of words explaining yourself.

Making it explicit has the benefit of making people actually think about who has control over the computers they are using.

Saying it's a service already makes the control explicit. Adding the word 'substitute' does nothing on that front.

It seems to me that people rarely think about the software behind a service, this is a mechanism to bring that secret software into people's minds and discussions.

Obvious, and emphasized, are very different things!