Why migrate to Drupal 8:
- Drupal 8 has new superior technology.
- Drupal 8 has new superior caching.
- Drupal 6 is no longer supported is can be vulnerable to hacking.
Drupal 7 is not as burning, though. Drupal 7 will be supported in the observable future. No burning need to upgrade from 7 unless there is some need for newer technlogy.
The latest version of Drupal, Drupal 8, does not allow an upgrade in the same way previous versions did. Instead, Drupal 8 has builtin support for an upgrade migration. The difference from the usual website migration is that you don't build your site before importing content. Instead, you import the data into a new Drupal 8 site, and then you make the necessary changes afterwards.
The course of Drupal to Drupal 8 migration goes like this. The existing content is migrated into a new Drupal site, and then that new site with the newly migrated content undergoes additional configuration and theming, as well as some new site building, since there are some configuration, like blocks positioning, etc, that is not migrated, and needs to be manual set post migration.