Why would North Korea admit that it was specifically a spy satellite? Sure, it would be impossible to hide the launch itself, but without their admission, the rest of the world could only have guessed at what type of satellite it was.
The value isn't it's (likely crap) utility, it's being able to say "Look at us: we're launching spy satellites just like technologically advanced countries! We're advancing our offensive capabilities! You'd better take us seriously!"
It's north Korea, even if it wasn't a spy satellite, everyone would assume it is anyway, so no sense beating around the bush.
And everyone would probably figure it out pretty quickly anyway based on where the orbit is, any signals they're able to intercept from it, etc.
And really, it's north Korea, they don't have a whole lot of needs or interests in satellites besides spying and military purposes. They don't really need communications satellites, their country isn't all that big and no one else really wants to talk to them and they certainly don't really want their citizens talking to anyone else either.
Which pretty much leaves observation satellites for things like meteorology, and monitoring crop yields and such, and since those kinds of purposes would probably have a camera onboard, they'd probably try to use it as as a spy satellite in addition to/instead of that primary purpose anyway.
Really though, their primary purpose for NK in to test missile technology somewhat on the down-low.
We can pretty easily track satellites in orbit now. If there was something new, we'd know. You can't hide that stuff anymore like you could in the 40s and 50s.
