Depends on the plastic to some extent - those made from cellulose may leave some NPK residue, but will also leave other, possibly more noxious residues. The term 'compostable' is a loose one with regard to plastics - the definition of 'home compostable' means the product breaks down into '...carbon dioxide, water, inorganic compounds and biomass...', which is all very well, but that covers just about everything in the universe, so it certainly doesn't rule out toxins being left behind, and when composted at home, methane will be produced. The term 'inorganic compounds' alone starts my alarm bells ringing, it's all terribly non specific.
If you were thinking of getting a load of 'compostable' plastic and having a dedicated heap with the expectation of using it on the garden, I'd strongly recommend you leave it just as a thought and not carry it out. Also bear in mind that 'biodegradable' does not mean the same as 'compostable' - some plastics are biodegradable, but that doesn't mean they'd be welcome additions to your compost heap. By and large, and as you suspected, biodegradable or compostable plastic is simply about disposing of the plastic rather than any useful by products from the process.