Your WDFW pass works at all area's controlled by Washington Department of Fish and Wildlife.  Which include nearly a million acres of land divided into 32 designated Wildlife Areas across the state. More than 150 Water Access Sites owned or maintained by WDFW provide boating access to lakes, rivers and marine areas in or near state Wildlife Areas.

To use State Parks and other lands controlled by the state, you must have the Discover Pass.  Which include nearly 7 million acres of Washington state-managed recreation lands – including state parks, water-access points, heritage sites, wildlife and natural areas, trails and trailheads.

Bottom line is.....if you only have the WDFW pass you are limited to just WDFW controlled lands.  If you have the Discover Pass....you can access not only State Parks and  DNR lands but also WDFW land. 

There was some talk about having to have a $30  Discover pass plus paying $7 to launch at Lake Sammamish

When you  got a fishing license you were given  parking pass to park at WDFW launch sites According to the website this will act as a discover pass.

Correct me if I am reading this wrong I did try to call the 1-866-320-9933 number but never could get a live person to verify this.
Go to this website http://www.discoverpass.wa.gov/exemptions/

then down load the PDF file Do I need a Discover Pass.

I also wonder would my WDFW pass work if I were to go to a park to hike or is it launch only.

My guess is the 2 passes look so alike that you would have no problem unless someone was to get out of his truck (not likely )as he drove by & read what your pass says.