As the core speed is increased, the heat that is created by the graphics processor too increased. This led to the addition of more fans in the body of the graphics card. First, they added one and then two and then now we have graphics cards with three fans to dissipate the heat.
It became impossible to add more than three fans due to the space constraints inside a computer cabinet. So the manufacturers instead invented a technology to daisy chain multiple graphics cards to work in tandem. Now gamers and scientists and engineers and architects and designers who were in need of huge graphics processing power started using multiple similar graphics cards in their computers to increase their performance. This also helped the graphics card manufacturer to increase their sales.
Cryptocurrency mining (for example Bitcoin) needed a lot of GPU power and that resulted in increased sales of graphics card the world over. In India in the year 2016, 2017 and 2018, graphics cards sales skyrocketed due to Bitcoin miners. A typical mining computer needed six (yes you read it correctly) very high-end graphics card daisy-chained to get the desired output. Cryptocurrency ban was proposed by the union government and is being objected by various forums and associations.