Wow I got royally lynched for this. Please see the accepted answer which illustrates the reasons for Programmers to exist pretty well.
I'd like to say for the record I stand corrected - Programmers clearly has its own role (albeit with a fair amount of overlap with SO) and its own community and identity.
The programmers.stackexchange.com FAQ defines its purpose:
Programmers — Stack Exchange is a site for professional programmers who are interested in getting expert answers on conceptual questions about software development. If you have a question about...
- algorithm and data structure concepts
- design patterns
- developer testing
- development methodologies
- freelancing and business concerns
- quality assurance
- software architecture
- software engineering
- software licensing
[you're in the right place]
The problem is that questions (and more importantly, answers) on all concept-based programming topics that I could think of seem to be much more prolific on StackOverflow than on Programmers:
- "algorithm" - on Programmers and StackOverflow
- "patterns" - on Programmers and StackOverflow
- "architecture" - on Programmers and StackOverflow
- "MVC" - on Programmers and StackOverflow
I suspect this is because the only people who care about code architecture problems are the same people who solve the day-to-day technical problems that should be posted on SO - so they naturally go there. There is no type of person who would exclusively use Programmers.
I would say this didn't necessarily matter - it's a good way of grouping questions by topic - except that it is much much harder to get an answer to a programming question on Programmers than it is on StackOverflow. I can post the same question (about code architecture) on Programmers and on StackOverflow and the StackOverflow one gets answered much quicker.
Given Stack Exchange's position on cross posting I therefore suggest that Programmers shouldn't exist. Either that or we need to find another solution so that people actually answer programming questions on Programmers.