Really, I so disappointed,May I know when to use image resource in nine patch format? 

For example, 
I have default image resource for user profile photo for image view with (50 \* 50)dp in phone sets. and i want to make the size of the image view in tablets (70 \* 70)dp.
Upon which criteria i use nine path image over making another image resource for table with higher (width,height)?

Another question, 
In some examples i see them exporting nine patch images for all resolutions (xxhdpi, xhdpi, hdpi, mdpi, ldpi) why they are making that?