The quick selection tool is one of the important tools you'll use while graphic designing. 

It's pretty useful when you need to cut out an object or a model (Which is pretty often believe me) . So while using the quick selection tool you need to make sure to convert the layer( from which you're cutting out the model)  or rasterize it. 

If you don't rasterize you won't be able to cut out the model. 

Now there will be some rough edges so you'll need to use the eraser tool to soften the edges so that it blends well into the background.

I'll cover the eraser tool in the next chapter. 

Sorry I know these chapters are short , I'll probably combine two tools in the next chapter.