Science Fiction Accelerator
Elon Musk is a character straight out of a science fiction novel. This is no accident. He has made it clear in multiple interviews that he has a deep love for science fiction. "Don't Panic" is gleefully emblazoned on the center console of the Tesla Roadster that is hurdling through space. He has actively used science fiction ideas to fuel the direction of his business life. Unlike other businessmen who prefer secrecy, Musk has publicly stated his goal: Mars. All of his current business projects have phenomenal immediate terrestrial applications, but they are also stepping stones on his path to Mars.
SpaceX is the most obvious example of Musk's Martian obsession. SpaceX has explicitly stated that one of their goals is to go to Mars. In order to get there, it first has to develop high powered re-usable rocket technology, which requires capital, man-power, and resources. Fortunately there are many companies and countries that want to put things into orbit. Offsetting high cost long range rocket development by developing an orbital delivery company is a brilliant way to create the technology for an inter-planetary fleet.
Tesla Is Not a Car Company
Tesla is most famous for producing electric cars, but it is not a car company. It is a flexible electric drive-train company. A powerful electric drive train is crucial for the multitude of vehicles that will be needed to develop Mars for human habitation. Construction on Mars will require front end loaders, backhoes, dump trucks, and other construction vehicles with powerful propulsion systems. The Tesla Semi is the first iteration of electric vehicles geared to an industrial application.
Solar Power and Batteries
Solar power will be key to providing energy for most if not all of the required power for all of the infrastructural and living needs on the red planet. The Tesla Solar Roof is a cheap, thin, highly-modular solar power generation system. Storing this energy on Mars will require batteries. The Gigafactories are solar-powered battery production facilities that are designed to be energy self-reliant. In Elon's mind, the Martian Gigafactory will be modeled on these terrestrial examples.
The Boring Company and Hyper Loop
The The Boring Company has been created with the stated purpose of making tunnels to provide relief to traffic congestion, but the real purpose is to create cheap tunneling technology. Developing efficient digging and tunneling technology on Earth is one of the keys to the colonization of Mars.
According to Wikipedia, the average temperature on Mars is -55 °C (-67 °F) with temperatures getting down to -153 °C (-243 °F) at the poles. On top of this inhospitable temperature, the surface of Mars is bombarded with radiation. Mars doesn't have the protection of a substantial atmosphere, magnetosphere, or radiation belts. People on the surface of Mars will be exposed to dangerous doses of solar and galactic-cosmic radiation.
Inflatable architecture is one way to create temporary surface level habitation environments but the thin layers of inflatable material won't provide much protection from radiation nor provide insulation against the cold. One long term alternative is to dig habitats and work environments underground. Theoretically these caves could be painted or sealed to create air tight living spaces. These could prevent many of the non-gravity related health issues that humans will face on Mars.
The Tesla Dune Buggy will likely be the first vehicle that the Martians drive, but these will only suffice for small scale transportation. Long distance heavy transportation will require larger vehicles. Most land-based resource transportation on Earth is performed via train. Trains are incredibly efficient and relatively cheap, but Mars may not be conducive to rail transport. Train tracks are made of steel to withstand the immense weight that is carried across them. Steel is incredibly strong but it gets brittle and can crack in the cold. This happens regularly on Earth and will indubitably happen far more frequently on perpetually frozen Mars. The martian dust will also likely impede mag-lev alternatives. Enclosing the train in a tunnel is one solution to these problems.
The Hyperloop is an interesting approach to a non-rail based train solution. The idea is relatively simple: build a tube and remove the air from it so that you can move a rail-less train rapidly without air resistance. Unfortunately, this isn't very practical on Earth because there is too much air pressure. Thunderf00t's excellent video essay dissects the numerous flaws with the Hyper Loop. Many of these issues are related to the fact that on Earth the air has to be pumped out of the tube and then it has to sustain a vacuum for the train to achieve a high continuous velocity. Creating a stable near-vacuum should be far easier on Mars. A subsurface tunnel based Hyperloop would remove the complexity of building an above ground tube.
Along with habitation and transportation another benefit of cheap tunneling technology is mining. Electrically driven machines that can chew through rock will also be useful for acquiring minerals, ore, and sub-surface ice deposits.
Elon may be claiming that he is building disruptive businesses on Earth but he is really constructing technological sandboxes to develop the technology to colonize Mars.