[Shader] normal vector and model matrix inverse

Hi guys !! I am learning "Specular Reflection" shader I got confused about this problem

 o.normalDir = normalize(mul(float4(i.normal, 0.0), unity_WorldToObject).xyz);

this is exactly what it have to be (theory)

but when i fixed it like this !!

 o.normalDir = normalize(i.normal);

THERE ARE NOTHING CHANGE !!

The 2nd one has to be wrong !!

Any answers for this ??? Thank for reading my question !!

Full shader code