about register reserved word or sementic in HLSL

Question

I'm studying HLSL cookbook. In that sample code, author used a lot of register sementic.

like this

cbuffer cbPointLightDomain : register( b0 )
{
    float4x4 LightProjection : packoffset( c0 );
}

cbuffer cbPointLightPixel : register( b1 )
{
    float3 PointLightPos : packoffset( c0 );
    float PointLightRangeRcp : packoffset( c0.w );
    float3 PointColor : packoffset( c1 );
    float2 LightPerspectiveValues : packoffset( c2 );
}

what is benefit of this?

actually I erased every register keywords and ran program.

the result is completely fine.

Answer 1

register exists to let you manually explicitly specify which register to use for a particular shader variable, in the event that you need to for some particular purpose or because you like to be explicit and not rely on how the compiler will pick registers (which may change over time).

It is optional, which is why you can remove it and everything still works.