When electric-car maker Tesla Motors'(TSLA 0.29%) launched Autopilot, an advanced driver-assist system, for its vehicles in late 2014, the company promised a richer driving experience than anything else on the market. And the company has delivered on these promises, now even moving beyond what it had initially promised by offering an added level of safety with a breakthrough over-the-air software update for the vehicle's radar sensor.

Model S. Image source: Tesla Motors.

But what is perhaps more impressive than the features Autopilot includes is the amount of miles Tesla's fleet is generating with the driver-assist system active. During Tesla's question-and-answer session with press over the weekend, CEO Elon Musk said the Tesla fleet has accumulated "roughly" 200 million miles on Autopilot.

Here are two reasons Tesla's rapidly increasing Autopilot miles are important.

A testament to Autopilot's effectiveness

For Tesla to hit 200 million Autopilot miles by now, the driver-assist system must be getting a lot of use. For context, it was less than two and a half months ago when the automaker hit 130 million Autopilot miles. At this rate -- and considering with the fact Tesla expects vehicle deliveries to increase substantially in the second half of 2016, Autopilot miles could surpass 1 billion before the end of next year.

But it's not entirely surprising Autopilot is getting a lot of use from Tesla owners. In comparisons of Autopilot and driver-assist systems from other automakers, it's evident there's a huge gap between Autopilot's performance and the performance of competing systems available in other production vehicles. Indeed, most driver-assist systems with automatic steering can't automatically change lanes when a driver wants to, making any sustained use of automatic steering impossible (unless you plan to stay in the right lane).

Tesla Autopilot. Image source: Tesla Motors.

But aside from this testament to Autopilot's driving experience, Tesla's fast-growing Autopilot miles could shape into a convincing argument for the safety of the system. Of course, the key word here is "could." While Tesla's current mortality rate of one fatality for 200 million miles driven with Autopilot is better than a mortality for about every 90 million miles driven in the U.S. and about one every 60 to 70 million miles driving in the rest of the world, the company will need to garner a longer stretch without mortalities than this in order to convince the public of its system's safety.

To be fair, as The Drive pointed out after an extensive review of Autopilot, the system is "definitely safer than a human driver alone, assuming you use it as intended." Even the tragic death of a driver who was using Autopilot in May would have likely been prevented if the driver was prepared to manually take control of the vehicle.

Further highlighting just how much customers are using Autopilot, Musk said during the company's press call this weekend that the fleet is now adding about 1.5 million miles a day, and the percentage of miles each driver is using Autopilot is increasing as people learn to use the system.

Improving fleet learning

Importantly for Tesla, each mile is more data for the company to use to improve Autopilot. With Tesla's "fleet learning" technology, the company processes vehicle driving data on its servers and uses algorithms to improve Autopilot in real time. And as Tesla captures more data and learns from it, fleet learning algorithms can continue to improve.

Model S. Image source: Tesla Motors.

No wonder Musk said during the Autopilot 8.0 press conference that he expected increasing data, improving software, and fleet learning to help Autopilot improve by "an enormous amount" in the future.

Interestingly, the takeaway for investors goes beyond validating an effective and improving Autopilot experience for Model S and X owners. More importantly, what Tesla learns about Autopilot can be applied to future vehicles. In other words, Tesla is developing and fine-tuning arguably what will be one of the greatest selling points for its first lower-cost vehicle next year; Tesla has said Autopilot hardware will be a standard feature on all Model 3 vehicles when deliveries of the $35,000 vehicle begin in late 2017. With Autopilot accumulating miles this fast, investors can rest assured the feature will be ready to impress when Model 3 hits the streets.