If Burundi goes 0 emissions tomorrow it hardly matters

If California does, it matters : - It shows it is possible, and increase political pressures on others - It create a pressure and market (California is big enough and Burundi or Alaska are not) for new solutions (technology based or not) that can be implemented elsewhere - It help decreasing the cost of many existing solutions, and everybody will benefits from it - It is possible to set up carbon tax at the border, increasing the incentive to follow California path

There will never be international agreement. No one will agree to put themselves at a disadvantage compared to everyone else. So greener technology will have to become the cheapest option or you'll have to commit mass murder to force other nations to submit against their will.