Bold typography is to specify the content that needs to be emphasised for example adding bold letters to your header usually makes the emphasis on your logo design and a company slogan. Thick fonts can also be used to demonstrate the visual hierarchy on screens. Adding bold fonts to your website is a way to attract more attention. Bold fonts are used to ensure the high readability. It is already scientifically proven that people tend to notice (and read) large-print content first. When combined with minimalist design, people can not help reading bold letters.
Serifs fonts are known as the most readable font for print but now serif fonts are also being used more and more for screen when San-Serifs are known to be the go to font for screens.
With bold type you are able to put images or patterns through them, seeing details through the letters. This wouldn't work if you use thin typefaces.