1) There's a lot of cases where blocking an entire domain is either excessive or insufficient -- important assets can be located on the same domain as ads. Adbl

1) There's a lot of cases where blocking an entire domain is either excessive or insufficient -- important assets can be located on the same domain as ads. Adblockers can target specific files and specific elements on the page. They also provide much better ease of use for the average user than updating a hosts file. 2) The list of ad domains, URLs, and elements has to be constantly updated. The plugin has to be kept up to date with changes in browsers. Advertisers occasionally come up with new techniques to bypass or detect adblockers, so those need to be worked around.