Constant float values in GLSL shaders - any reason to use uniforms?

Question

I'm looking at the source of an OpenGL application that uses shaders. One particular shader looks like this:

uniform float someConstantValue;
void main()
{
    // Use someConstantValue
}

The uniform is set once from code and never changes throughout the application run-time.

In what cases would I want to declare someConstantValue as a uniform and not as const float?

Edit: Just to clarify, the constant value is a physical constant.

Answer 1

Huge reason:

Error: Loop index cannot be compared with non-constant expression.

If I use:

uniform float myfloat;
...
for (float i = 0.0; i < myfloat; i++)

I get an error because myfloat isn't a constant expression.

---

However this is perfectly valid:

const float myfloat = 10.0;
...
for (float i = 0.0; i < myfloat; i++)

---

Why?

When GLSL (and HLSL for that matter) are compiled to GPU assembly instructions, loops are unrolled in a very verbose (yet optimized using jumps, etc) way. Meaning the myfloat value is used during compile time to unroll the loop; if that value is a uniform (ie. can change each render call) then that loop cannot be unrolled until run time (and GPUs don't do that kind of JustInTime compilation, at least not in WebGL).