From what I can tell, Improved Session Engine Because new session layer is leaner and faster. Thus, the older one wasn't as lean, and was slow.

I believe you're right. When using $_SESSION it uses memory that a large public app cannot always afford.

"file" would be similar to native.

Why recreate a file/array session handler? Control. Knowing what's going on. Easier to debug when people raise issues. When things go wrong related to core PHP, php ini, or your server technology; Apache config, Nginx, etc.... it would be a head-ache. Controlling it all avoids having to work out who is to blame for a session bug.

Somebody over at reddit pointed me to a comment from Taylor Otwell that gives one of the reasons behind the change.

@christoferd it looks like you were correct - it's a matter of control. Specifically, the ability to control how the sessions were managed as part of Laravel's request/response cycle. Using PHP's native sessions meant that session cookie headers were sent outside of Laravel's response.