THink of Render Engine as a completely stripped down version of AE, and all it is there for is for the Watch Folder feature. If you have multiple computers networked together, you can install Render Engine on all of them and use it to set up a render farm.
I actually use it all the time because you can set up a Comp to Render in Render Engine while you continue working in the Full version of After Effects on the same computer (if you are on a Dual processor mac, which you are.)
Now as for why that’s coming up instead of the Full version of AE? How are you opening After Effects? Are you double clicking on a project file? Because that can be associated with the wrong application. Go into The Applications folder and into the AfterEffects folder and there should be one version of the Application that has an RE after it, that’s the render engine version. So you want to double Click the other application. If not, try doing a search for After Effets and see if you can find 2 applications.
Hope that helps