Scenario
Joe is a student taking Japanese 2 at MIT. The class uses the Genki 1 textbook, and has studied the grammar, characters, and vocabulary up to Chapter 10. The textbook has taught Joe some useful grammar and vocabulary, but not enough to read most of the Japanese-language content on the web or in books. The textbook contains some example sentences for reading practice, but Joe has already read them and still doesn't feel like he's mastered the material. Joe want to practice reading additional sentences to solidify his understanding of the grammar, characters, and vocabulary that have been taught in Genki 1 up to Chapter 10.
Following his friend's suggestion, Joe heads to the Reading Practice website. The website asks him the language, textbook, and chapter in that textbook he's studied up to, and Joe specifies that he's studying Japanese from the Genki 1 textbook, and has studied up to Chapter 10. Then, Joe gets displayed a list of novel sentences in Japanese that use the grammar and vocab that he's learned, and he practices reading them.
After reading a few sentences, Joe encounters the sentence りんごを食べたいです。 He's unfortunately forgotten what the vocabulary word りんご meant, but he doesn't want to look it up in a dictionary, so he clicks on the word, and the interface tells him that it means "apple". Joe then see 食, and while he knows it has something to do with eating, he doesn't know how it should be pronounced in this particular context. He therefore asks the system to romanize the sentence, and seeing "Ringo wo tabetai desu", he now knows that 食 is pronounced "ta" in this context. However, he still doesn't understand what the sentence means, hence he asks the system to show him an English translation. Seeing the translation "I want to eat an apple", it dawns on him that he's forgotten that the -tai verb conjugation is used to indicate want/desire. He wants to practice reading this sentence pattern more, hence he asks the system to show him a list of sentences that use the -tai verb conjugation.
After reading a few more sentences that use the -tai verb conjugation from the list, Joe notices a sentence that isn't classified correctly. He had been searching for sentences using the -tai verb conjugation, but this sentence doesn't include it. He therefore removes the "-tai verb conjugation" tag on the sentence, so that it won't be displayed to users searching for that type of sentence.
A few years later, Joe is now fairly fluent in Japanese. He'd like to contribute back to the website that helped him practice reading Japanese back when he was starting to study Japanese. Hence, he logs in and enters a new sentence. He types in, ビールを飲みませんか。 and the English translation, "Would you like to drink beer?". The system helpfully uses some heuristics to suggest that the sentence be tagged as having a "-masen verb conjugation", and Joe additionally adds the "making an offer" tag. Thereafter, students looking for these types of sentences, either because they're explicitly searching for sentences with that type of tag, or the textbook chapter they're studying has covered the usage of a sentence type using that tag, will see Joe's newly added sentence.