Question2Answer ( known as Q2A ) is a different project than WordPress and there is no relation between them but Q2A provide feature to integeat with WordPress user database and so once you have integreated it with WordPress Q2A will use WordPress user information to authenticate by using and comaring WordPress user role and permission with Q2A user role and permission.
Technically there is no relation of WordPress theme with Q2A theme and both are independent to each other. But You can find similar WP theme as Q2A Snow theme here http://store.q2amarket.com/store/products/question2answer-wp-pack/ this pack includes Snow theme for WordPress and Widget, shortcode and function for WordPress to display some Q2A activities on WP website.
That is difficult to say any reason why people integreat Q2A with WP. But most comon reason is to integreat same user database/authentication which allows user to use same userid and password for both system Q2A and WP. Q2A also offers same Single Sign On for other CMS as well.
I hope you will get more answer from community members...