Is the "Save for Web" is useful for resizing the picture without loosing quality?

yes it helps in various ways to keep the actual quality of the picture

yes, it is useful for web and also not not loosing the quality of Image.

Absolutely. I mean it reduces the size to just few KBs without leaving that much effect on the picture quality. 

Yes, It is very useful specially when we use images for web we can get benefit from this command. 

I think no,

but you can save as PDF.

Yes, and it is save the same size i want automatically in a pro way