The initial impulse behind these was to offer some protection against the arbitrary expropriation of foreign investments by less-than-democratic governments. But now corporations have realised that they can use the investor-state dispute mechanism to challenge all kinds of legitimate but inconvenient decisions in any signatory nation. Here's a good example of how this provision is being invoked to contest a refusal by Canadian courts to grant a patent on a drug,:
Eli Lilly and Company has initiated formal proceedings under the North American Free Trade Agreement (NAFTA) to attack Canada's standards for granting drug patents, claiming that the denial of a medicine patent is an expropriation of its property rights granted by the agreement. The investor privileges provisions included in NAFTA and other U.S. "free trade" agreements (FTAs) empower private firms to directly challenge government policies before foreign tribunals comprised of three private-sector attorneys, claiming that the policies undermine their "expected future profits." Eli Lilly's move marks the first attempt by a patent-holding pharmaceutical corporation to use U.S. "trade" agreement investor privileges as a tool to push for greater monopoly patent protections, which increase the cost of medicines for consumers and governments.
Canada Denies Patent For Drug, So US Pharma Company Demands $100 Million As Compensation For 'Expropriation' | Techdirt