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@TSS-Graviser There isn't much of a big "reason", it's simply how we define the "measure" of an "arc".
If you think about it, arcs aren't really "angles", right? But, we find that it is very useful to just "say" that an arc has an "angle measure". But why, and what would it even represent?
An arc is just part of a circle. So, we can think of the "angle measure" of an arc as how much the arc goes around the circle. For example, the full circle is 360 degrees - if you stood on a playground and drew a full circle around you with a stick, you'd spin a full "360 degrees". If you only went a quarter of the way around, you only draw "90 degrees" of an arc.
As you can imagine, how much you go "around" is exactly the central angle you make!
So why is this useful? Off the top of my head, a good reason is that using "arcs" instead of "central angles" can be a good shortcut, and can be easier to think about. For example, inscribed angles along the same arc have the same measure!
And we can say that the measure of each of those inscribed angles is half the measure of the intercepted arc, and we don't have to draw in the central angle.