Mainly hased files are used for lookup perpose. for example your looking for the data whether the entry is their or not. if it is thr do something otherwise something else
Hashed file is used for lookup process. Beside that, Hashed file can also be used if we want to get Distinct data from a source by using the column as key. Sample, I want to get list of country from Customer records. One possible solution is I create a Hashed file and using Country as its key. Then I can get distinct value of Country.