From Apple’s press release:

Apple and IBM (NYSE: IBM) today announced an exclusive partnership that teams the market-leading strengths of each company to transform enterprise mobility through a new class of business apps—bringing IBM’s big data and analytics capabilities to iPhone® and iPad®.

The landmark partnership aims to redefine the way work will get done, address key industry mobility challenges and spark true mobile-led business change—grounded in four core capabilities:

• a new class of more than 100 industry-specific enterprise solutions including native apps, developed exclusively from the ground up, for iPhone and iPad;
• unique IBM cloud services optimized for iOS, including device management, security, analytics and mobile integration;
• new AppleCare® service and support offering tailored to the needs of the enterprise; and
• new packaged offerings from IBM for device activation, supply and management.

Tim Cook speaking to Re/code:

“If you were building a puzzle they would fit nicely together with no overlap,” Cook said of the relationship. “We do not compete on anything. And when you do that you end up with something better than either of you could produce yourself.”

That sums up why this deal is so good for both companies. Since there is no overlap, both will benefit equally in making this work.