Dictionaries are essential for natural language processing, especially when it comes to removing stop words and normalizing text. By using dictionaries, search engines can improve relevance and accuracy by breaking down complex words into their constituent parts, such as atoms or segments. This is particularly important in languages that use accents or have complex grammar rules, where normalization techniques like accent removal and segmentation are crucial. Custom dictionaries can also be used to override default settings and add customer-specific words to the search engine's database. By integrating custom dictionaries with static dictionaries, search engines can prioritize dictionary lookups and improve overall performance.