Hi.
To be honest you could light the whole scene with one environment light and nothing else. Have no global backdrop effects, use the environment light panel as above image, to add a backdrop gradient first and then use a physical sky second and it would light the scene.
Having the two lights in the scene with the distant one as well, allows you to link this distant light to the environment light that is using the physical sky so you can better position the location and elevation. It is far easier and quicker to tweak than using the Hosek settings to achieve same thing. Notice if you have one environment light in the scene, the azimuth elevation options are available to tweak position and height. If you add a distant light but in the physical sky settings for the environment light, link item to itself the options for azimuth/elevation become greyed out. However when you link the physical sky from environment light to the distant light the options are available but don't seem to change anything, that is because it is expected that you would just adjust heading and pitch of light from the normal transform gizmo.
As far as your question why use distant light type as distant and not as sunlight type. This is to give you flexibility. The addition of distant light, not only allows you to tweak position a bit easier but the reflections themselves. This second light allows you to either use a physical sky type reflection or you could set your own reflection colour and make different looks. Think of the second light, the distant one, as a tweak to the reflection colour and intensity of your highlights in the scene without affecting the overall ambience the physical sky is giving you.
So to keep things simple. Environment light for overall light ambience, a second distant light for reflection/highlight intensity tweaks or mood looks. If you felt your environment ambience was nice but your specular highlights needed a boost, a second distant light allows you to keep the nice look of your environmnet and boost up your highlights or dial down the highlights as needed or blend in some extra colour.
With the second distant light then, you could of course make it a physical hosek or just as a standard distant light type in settings. This will affect the specular look of the sun in the reflections for more dramatic look or just to add some personal tweaks.