When: 1976. Why: First camera with the microprocessor. First SLR to be sold in millions of units.
The second "why" is actually coming from the first one. The camera was really easy to use (due to the CPU advantages), so many amateur photographers were excited to get it. More than 5 million cameras were sold in total. These days almost every film camera has a microprocessor in it. Also, microprocessor is one of the most important parts inside of all the digital cameras on the market.