LanguageTool can be used to validate user input, for example to check if the user contributed properly capitalized song names to an online database. For this use, you need to create some custom rules and use them, for example via the HTTP Server interface (you need to configure the server to run only the custom rules then) or the command-line.
A more complex example of free text validation might include checks on bilingual content. For example, if you have a database that has some localized text along with original text (like titles of books and their translations), you might additionally check if the title is capitalized differently for a target language. For example, Polish capitalization rules are completely different from English ones, and data, if input by Polish or English speakers, might end up improperly capitalized.