Every release of Drupal requires its own version of Drush. There are some, but not many, changes for Drush moving from Drupal 7 to Drupal 8. Drush 8 can be used to control the new configuration management system.
With phone in hand, laptop in bag and earbuds in place, the typical user quickly scans multiple sites. If your site takes too long to load, your visitor is gone. If your site isn’t mobile friendly, you’ve lost precious traffic.
Your time is limited. By the time you finish this sentence, someone will visit and abandon your website. Unless you give your visitors a reason to stick around, they're gone. High bounce rate is an indicator of visitors' lack of interest.
A cool thing about Drupal is that you can modify and add to the core offering. Think of D7 and D8 as basic buildings, and modules as all the things you add to buildings to make them more useful.