A standard feature in email clients today is the ability to search your emails for certain content.  This can be useful if you want to find all your emails from a certain person, or about a certain subject.

With some email clients, such as Horde, the search feature is more robust and allows you to tell the client not only what to search for but where to search:

Other clients, such as Thunderbird, have a general search bar and will search all your emails for any word string of your choice:

No matter what style of search functionality your client offers, it is a great tool to use if you need to find an email quickly on a certain topic without looking through all your emails.