This bacteria was the first organism to evolve the capacity to take H₂O and CO₂ and convert them into glucose via photosynthesis. At this point there were no oxygen-breathing lifeforms and so all of the oxygen byproduct was effectively toxic. Cyanobacteria subsequently adapted to be able to use this oxygen via respiration.
Algae adopted this capacity and came to dominate and the suppression of cyanobacteria, which produced toxins harmful to other life, allowed for the evolution of more creatures, including humans.
However, the warming of the atmosphere and creation of CO₂ is re-establishing an ideal environment for these cyanobacteria, and unless we get the problem under control many lifeforms will start dying from the toxins they create.