As uv attribute is texture coordinate assigned to each vertex . What is the point of 3rd coordinate in uv attribute ? Is it something like width or scale ? then how does that scale value works ? somebody help me out

it's just a 3rd dimension

uv doesn't need to store just flat 2d coordinates, it can be full 3d coordinates similar to what v@rest attribute is commonly used to
so you can use it to sample 3d procedural textures for example