"Apple innovates like no other company on Earth, and we are not taking our foot off the gas," said Apple CEO Tim Cook on the company's earnings call Jan. 29, 2019.

Apple has upped its commitment to health tech with recent partnerships with insurance companies and its new sensors to track heart health. Cook recently said in an interview with CNBC's Jim Cramer that he hopes it will be the company's "greatest contribution to mankind."

But they do get passed around the company, and sometimes they inspire real action inside. In particular, they've helped guide the company deeper into the health care field.

In 2017, two years after the Apple Watch launched, the company released a marketing video featuring people who wrote Cook about how the product had helped them. It included a man who used his smartwatch to call emergency services after a car accident, a father whose daughter has diabetes and now integrates her blood sugar readings and shares it with family-members, and so on.

But the letters are used for much more than marketing. They can also influence product decisions.

According to people familiar with how the process works, Apple's Cook has an assistant whose job it is to read the mail, forward some to him for personal attention, and share others to a group distribution list of executives on the relevant teams. They forward the letters to their reports, and so on down the chain. Many of these "Dear Tim" letters are ultimately passed around by rank-and-file employees, according to one current and two former employees.

The letters have had a particularly large impact on Apple's health team.

After the Apple Watch launched in 2015, the company promoted a variety of features on it, including communications, entertainment, and health and fitness tracking.

But then the missives started pouring in from users, describing how the device alerted them to potentially serious medical conditions and even saved lives. After this, Apple began shifting the emphasis of the watch more toward health features.

One early story involved a teenager named Paul Houle, Jr., who noticed his heart rate was double where it should be after football practice, prompting him to seek medical care. It turned out that he had a serious health condition, which was already starting to cause damage.

Tim Cook personally called Houle and his family after he got wind of the story, ABC News reported. Apple declined to comment at the time.

One former employee described the letters as a surprise, as no one expected the heart rate tracker to pick up on irregularities that indicated more serious problems. Another former employee said the heart-related letters showed Apple could have a much more positive impact on health than anybody at the company had previously realized.

The letters have also been helpful in maintaining employee morale. One person recalled how the letters were particularly meaningful to engineers, who didn't have an external-facing role and weren't regularly meeting with users. Apple is notoriously secretive, so many employees can't talk much about their work, even to close friends and family.

The topic of the "Dear Tim" letters came up again at a rare public event for Apple's health team earlier this month.

"It was a new day for Apple when the letters started coming in," noted Julz Arny, who works on special projects for fitness at Apple, during a recent panel discussion at the company's San Francisco Apple Store.

"People weren't writing love letters (in the same way) about Calendar," Arny explained. "Everyone internally finds that to be so exciting."