Twitch announced changes to Profile Pages last July 27 as they put the new features on Beta. Today, the new Twitch Profile Pages are live to all registered users.
The redesign is intended to improve social discovery on the platform. Previously, when you go to a channel and there is no livestream, you are simply directed towards that offline channel. Twitch will now redirect users to a streamer's profile so that they can learn more about said streamer. Users will continue to go straight to the livestream when it is available, so these changes will not affect the business of streaming itself.
Under the new profile page layout, Your livestream channel, as well as links to your most recent videos, are front and center. Your panels have been moved to the far right to reside with the chatbox, and your profile information stays at the far left. In fact, the profile information tab on your profile and channel will look exactly the same, to make switching between the two consistent and painless. The flip side to this is your personal information now resides behind a tab.
Overall, these changes make Twitch very similar to YouTube, and I'm sure Twitch wants its users to treat their platform in the same way. Although seemingly minor, Twitch hopes these small changes will encourage more activity between fans and streamers, ultimately leading to more views and subscriptions.
Really, the question here isn't why Twitch did this, but why they waited so long. Twitch has also recently partnered with Steam. Slowly but surely, Twitch has been active in extending its influence in gaming.