You might want to watch the video at the below link for a quick explanation of what prefabs are:
When you import an external 3d model file like a fbx or a .blend or an obj, it is imported in its native format. When you drag and drop it in your hierarchy thus using it in your scene you are using an instance of that original object. Once you create a prefab of this object, the prefab in always referencing back to the original file that you earlier imported. So if you delete the original file all of the data(mesh,textures,etc.) will be destroyed as well, leaving an empty prefab.
Prefab is a asset type that allows you to store a GameObject complete with components and properties. The prefab acts as a template from which you can create new object instances in the scene. Any edits made to a prefab asset are immediately reflected in all instances produced from it but you can also override components and settings for each instance individually
When you drag an asset file (eg, a Mesh) into the scene, it will create a new object instance and all such instances will change when the original asset is changed. However, although its behaviour is superficially similar, the asset is not a prefab, so you won’t be able to add components to it or make use of the other prefab features described below.
Leave it in your project. Unity represents external assets as something, in that case it’s a prefab. If you were to create a material using a texture, would you delete the texture? No, because it’s just a reference.