Search advertising is sold on the basis that advertising displayed next to search results for a specific keyword or next to content about that specific keyword will be effective in targeting users interested in products related to that term. Due to differences in context and the simplicity of this environment some totally different terms would attract the same advertisements if negative keywords were not possible.
A double glazing company would not want their advertisements for replacement windows to run alongside advertisements for the popular operating system. Negative keywords such as ‘software’ could be used to ensure for example that these advertisements wouldn’t show next to results for the term ‘windows software.’