When you combine them, 用 (to use) + 完 (to finish), you get the clear and direct meaning: the action of using has been completed, resulting in the item being fully depleted.
While 用完 is a practical term, its structure reveals a key feature of the Chinese linguistic mindset: an emphasis on the result of an action. In English, we might say “I used the salt,” and the context tells us if it's all gone. We can add “up” (“I used up the salt”) for clarity, but it's optional. Chinese is often more explicit. By attaching a result complement like 完 (wán) directly to the verb, the sentence leaves no ambiguity. 我用完了盐 (Wǒ yòngwán le yán) doesn't just mean “I used the salt”; it specifically and clearly means “I used the salt and now there is no salt left.” This grammatical structure forces the speaker to be precise about outcomes. This focus on results and completion is a subtle but important difference from English, where the action itself is often the primary focus of the verb. For a learner, mastering result complements like 用完 is a major step toward thinking in a more Chinese way, focusing not just on what you're doing, but on the outcome you've achieved.
用完 is an extremely common, everyday term used for any consumable or finite resource.