If you have a late 2015 model, then you have a “Display P3” screen. In that case you can set it to Display P3. Display P3 is different from Cinema (or regular) P3 because its white point is different.
The advantage of setting it to sRGB is that you can view images the way that people with sRGB displays would see them (which is most of the world). AdobeRGB and P3 are about the same size, but each has colors the other doesn’t have (basically if you rotate AdobeRGB, you get P3 and vice-versa). Setting your display to AdobeRGB will clip colors on your display, but not let you preview images as a AdobeRGB monitor might show them.