You ask an interesting question. To start, it would be helpful to remember that a patent is in essence, a social
contract between the inventor and society.
Since it has long been understood that new invention stands on the shoulders of older invention, a system is needed to encourage an inventor to share his secrets so society (other inventors) can learn from them and innovate further (and faster). Yet in a capitalist society where monopolies are not considered beneficial to a free market system, it is also understood that without protection from the risks of experimentation and innovation, the inventor will not share his secrets. Hence, patent law provides a term of legal monopoly in return for full disclose of innovation. But this point is also important to remember: A patent is tool to prevent others from using your invention.
The case of Tesla is interesting and I don't necessarily agree with the comment above regarding "good faith". Musk's comments invite others to use (license) Tesla patents without benefit of payment and relying upon that, a company might successfully use this in any infringement proceeding. As a result, while Tesla is not giving away it's patents (they can sell these and a purchaser may decide on a different strategy) Tesla is inviting others to use its patents license-free.
Tesla's motivation for this action may be quite simple, assuming the patents in question rely on electric propulsion etc. Encouraging additional competition into the electric vehicle business necessarily creates more electric vehicles on the roads. More electric vehicles means more charging stations and perhaps the encouragement of faster charging techniques. This in turn will create greater demand for electric vehicles and Tesla stands to benefit, i.e., "a rising tide lifts all boats".
But Tesla is not a start-up. Those that may wish to invest in a start-up want every advantage possible, including intellectual property that might be quantified. Those that want to innovate do not relish sharing their secrets freely and want the protection that a patent affords. In my opinion, IP protection in the form of a patent remains important for a start-up. Perhaps more so.