Corner clamps for a frame, What is this for?

I was making a frame to put around a mirror in my daughters room last night. using 1"x 3" pine around a 60" x 18" mirror.
When it came time to glue up the frame, I could not find the strap clamp I usually use. Digging through the tool box I came across some corner clamps I inherited but have never used.
One of the clamps has a guide for a saw blade that would run right down the 45 degree cut where the corners meet. The other 3 clamps do not have this feature.
I'm guessing by clamping the two pieces of wood together and running a saw through this guide it would make a perfectly fitting joint?????
I had used my power miter saw to cut the 45's on the frame and using the clamps, they did not align very well, not a problem in this project as the frame will be painted so I just filled the imperfections in the joint.
I'm going to examine this closer when I get home tonight.
1. I used the 45 degree stops on the miter saw and as I haven't fine tuned them I doubt they were accurate right out of the box.
2. I'm also not sure that the clamps are exactly 90 degrees???
3. What's the better method of cutting 45's? A - adjust the stops on my miter saw so they are exactly 45 degrees. B - Cut all the 45's and then put them in the clamp with the saw blade guide and run a saw blade through which would even off the two faces so they came together. I guess I would have to repeat this for all 4 corners...
I plan on making a few picture frames and would like to get a nice miter on the corners.
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