Apparently this wasn't a thing prior to today.

If you are an iOS user that frequently watches Twitch streams, you'll want to pay attention. Starting today, you will now be able to subscribe to your favorite partnered of affiliated Twitch streamers directly through iOS.

This is made possible with today's launch of iOS Sub Tokens. iOS users will have to purchase these Sub Tokens through the Twitch app. Each token can be redeemed for a one-month Tier 1 subscription to a streamer. You can renew your subscription by redeeming one token per month, up to 12 tokens at a time. They function like normal subscriptions made through the desktop and give you sub badges, sub emotes, and the lot.

The difference with these Sub Tokens seems to be the cost. Each token will run you $5.99 (USD) in order "to account for mobile app store fees." This is about a dollar more than a normal subscription cost.

Twitch is offering a special offer for the rest of the month to celebrate the launch of these Sub Tokens. From now through October 31, you can buy two Sub Tokens for $8.99 (USD). After those two months, I'd maybe suggest just subscribing through the desktop whenever possible so that you can save that extra dollar. Yes, it's only a dollar, but that'll add up over time if you keep subscribing to various streamers every month.