Twitch isn't going into tremendous detail at this stage, but it notes that streamers can provide special emotes to higher-tier subscribers. You could also see new "subscriber roles" at these levels. All of this is optional, though, so you won't necessarily be pressured into paying more. Partners, meanwhile, can unlock emotes and hit milestones faster -- a $10 subscription is treated like two subscribers, while $25 is worth six.
The beta will take a little while to spread, reaching every partnered streamer within "several weeks."
This is both a competitive move and, arguably, a vital one for the livestreaming crowd. If Twitch can promise more rewards to frequent streamers, it might keep them from switching to Microsoft's Beam or YouTube. Also, the simple reality is that there are many people who stream on Twitch for a living and have been hampered by the current $5 subscription ceiling. Unless you're wildly popular, it's difficult to make a good income without resorting to alternative funding sources and sponsorships. Only a fraction of subscribers are likely to spring for the higher amounts, but it could encourage more people to treat Twitch streaming as a job, not just a hobby.