A forward grip is a handle that you attach to your replica and ensures that you can keep your replica stable and firm in your hands. There are different types of grips that each provide a different attitude. The preference of position is therefore very different per person.
When you have a grip you have to keep in mind what kind of replica you have. if your replica has no rails, you can often use a grip with keymod. It is also possible to attach a rail mount to some replicas so that you can still install your chosen grip on your replica.