Photosynthesis is extremely important to most life currently on earth. Humans cannot survive, currently, without photosynthesis. In fact, most life on earth cannot survive without photosynthesis. Photosynthesis is needed to provided energy-rich food for almost all organisms on the planet. While there are other sources of energy that are not directly dependant on sunlight, they are quite limited in number and location.
If photosynthetic organisms all died out suddenly, then all of the other organisms that relied upon them would be under great pressure to adapt in a short period of time. This would likely cause a large proportion of the species on earth to go extinct.