In a object where to use raycasting?

Particularly where raycasting in a game is used? How it is used? What are the characteristics of raycasting?

It kinda depends on the game genre. For example, if you were creating a shooter or an FPS, you’d use Raycast to determine where the player is pointing, and use that contact point to instantiate bullet holes, particles, etc. Raycasting basically lets you cast an imaginare (though you can make it visible) line in a certain direction that will collide with any object holding a collider, and will help you store where that object is, and the exact point of contact.

About how it is used, you have quite a few examples in the Unity Documentation: Unity - Scripting API: Physics.Raycast

Also there are plenty of video tutorials in youtube about this specific and very useful feature.