Insurer Aetna Inc.  (AET)  is making Apple Watches available to employees of its largest corporate customers, according to a source familiar with the matter, highlighting the importance of health and fitness to Apple Inc.'s  (AAPL) line of smart watches.

It's not known how many of Aetna's 23 million members will get the watch, or whether it is subsidizing the devices, but the move suggests that a vibrant future for wearables in corporate wellness plans. Aetna already offers an Apple Watch to its 50,000 employees as a wellness benefit. CNBC first reported on Monday that Apple and Aetna were negotiating a plan to offer free or discounted Apple Watches to Aetna's customers.     

While health and fitness apps may not yet drive massive sales of the Apple Watch, which Apple does not yet break out in its results, there is clear interest. Among more than 2,000 North American consumers that 451 Research surveyed in April, 64% of those who plan to buy a smart watch said health and fitness monitoring were the most important functions. Water resistance, the ability to send and receive texts and wireless connectivity were next highest, selected by 31%, 29% and 25% of respondents, respectively.

"Health and fitness are quietly becoming killer apps for Apple Watch," Neil Mawston of Strategy Analytics said in an email. 

Mawston suggested that more agreements like Apple's deal with Aetna will follow. "Corporate wellness is a fast-growing market and Apple Watch can benefit employers by developing healthier employees," he said. Apple and others will have to address financial, legal, ethical, cultural and logistical challenges along the way. "For example, there remains some debate about whether smartwatch sensors are accurate enough in the near term to provide reliable data or algorithms for enterprise-grade employee monitoring," he said.

Apple boss Tim Cook has been bullish on health and fitness apps for the smart watch, noting that the device is "motivating [users] to sit less and move more" during a third fiscal quarter earnings call in early August. Upgrades coming in Watch OS4 will allow the device to serve as a personal activity coach, while Apple's GymKit program will connect the watch to cardio equipment, he continued.

The current Apple Watches require proximity to an iPhone to connect to the Internet, limiting the appeal to Aetna customers or others who own phones with Android or other operating systems. The next iteration of the iPhone reportedly will have its own wireless connection, a boon for non-iPhone users.

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