The three settings are all useful -
The 0-45 setting is for when you need to quickly change from 0 cuts to 45 cuts. The stops are set so you don't need to fine tune it.
The 45-45 setting is for when you need to switch from 45 cuts at each side.
The 47-47 allows travel the full extent, so you can set angles over 45.
Imagine there was no knob, and only 47-47 setting - it would mean that, every time you wanted to set the saw to exactly 45, or 0, you would need to use the fine adjust to set it. It would soon get tiresome! With these three settings, if you want to set it to exactly 45, you just turn the dial to the relevant setting. To reset it to 0, just tilt the saw left, dial the 0-45 setting, and move it back upright. When it hits the stop, your saw is at 0.
Hope that explains it adequately.
JRB