What is the role of “Site Speed” in GA? How it helps?

Hi,
I checked GA & found a new factor 'site speed'. i went through all points and metrics mentioned in that. In most of terms google mentioned 'samples' or 'sample set'(https://developers.google.com/analytics/devguides/reporting/core/dimsmets/sitespeed). What does it mean. Is there any ideal data for each and every metrics Google mention? if yes, than what is that?
What is the ideal page speed Google considered as OK.
Lots of other questions are still in row.. Looking for some relevant answer…
What is the role of "Site Speed" in GA? How it helps?

See original post by naveenjosh