I view schedules around software projects as a few different things:
- Public expectations for specific deliverables that your customers can align to, many times with a calendar unique to each customer.
- Internal carrot/stick for managing constraints around active feature development. If you have an infinity budget (film/AAA gaming), you might be able to run less-bounded parallel efforts as noted elsewhere in this thread.
- Orthogonal operational concerns (planned outages, et. al.)
- Roadmap for strategic product development that the investors can think about
So, when someone says something to me like "are we still on track for end of the week?", I have to provide an extremely qualified series of responses and ask more questions.