This morning, Tesla Inc. (TSLA) CEO Elon Musk announced on Twitter that all Tesla Model S and Model X owners are about to get a free upgrade - another one, that is.

All X & S will soon work with no key if your phone pairs w Bluetooth, signal strength is high & you enable feature

— Elon Musk (@elonmusk) May 10, 2018

When that update comes out, it'll just be the latest in a string of big over-the-air software updates that Tesla has delivered to owners in recent months.

And as consumers become increasingly likely to purchase electric vehicles in the years ahead, Tesla's policy of providing free feature-adding software updates to its existing fleet gives the company a major edge over other manufacturers.

Feature improvement has been a core principle of Tesla's cars for a while now - the firm sold its Autopilot system largely on the promise that, as Tesla's self-driving software continued to improve, cars would be able to leverage their existing hardware for full self-driving. But Tesla's feature enhancements go well beyond Autopilot.

In recent weeks, Tesla pushed a completely re-engineered navigation engine (currently in public beta) to owners, promising better routing and more accurate response to real-time traffic conditions.

Other software updates have provided a "camper mode" that allows keeping the climate control active for prolonged periods with the vehicle off; automatic windshield wipers for the Model 3; cabin overheat protection; additional car controls from Tesla's phone app, and more.

Some investors have called Tesla's cars bigger, more expensive iPhones - and in a way, they're totally right. Just like new iOS releases from Action Alerts PLUS Charitable Trust Portfolio holding Apple Inc. (AAPL) unlock new functionality, Tesla's new code makes your car do more.

The difference is that a four-year-old Model S doesn't have any of the battery degradation or sluggishness of a four-year-old iPhone.

Not a lot is free in the car world. Conventional automakers are notorious for charging big markups for features that cost them next to nothing. $800 navigation update DVDs come to mind.

Meanwhile, Tesla's steady stream of feature improvements and updates on existing vehicles is a huge value-add for consumers thinking about making the move to an electric vehicle and ensures that Tesla's electric car market share remains well defended in 2018.

All you need to know about Tesla here.

This article is commentary by an independent contributor. At the time of publication, the author was long AAPL.