Use can use this when the person is able or allowed to use the thing you're pointing at. You can use this laptop if you want.
Giving permission? You can use this room until five. Stating ability? You can use this pen to sign the form. Context decides which one your sentence is doing.
When you're granting someone permission, the condition is that you hold the authority and you're giving it: You can use this desk while I'm at my meeting. Here you're the one allowing it, so the sentence comes from the speaker who is granting it.
Permission from a rule rather than a person: Visitors can use this entrance on weekends. You're stating what's allowed, so the listener knows they don't need to find someone and ask.
Conditional permission: You can use this printer as long as you bring your own paper. The same sentence gives you permission and sets the limit: you can print freely, but only with your own paper. The listener hears the yes and the limit in one sentence.
Requesting permission from whoever controls the item: Can I use this computer for a minute? You're not making a statement here. In a question, the phrase becomes a polite request, and the other person decides whether to say yes or no. The computer is theirs to grant or refuse, so asking with can use this hands the choice over to them.
When you're describing what someone's able to do, use can use this for things that genuinely work: You can use this app offline once you've downloaded the maps. The sentence works because the offline feature really does exist once the maps are downloaded, so you're telling them what works, not what's permitted.
Ability tied to a situation: You can use this shortcut to reach the station in ten minutes. The listener is capable of it as long as they follow the route you're describing, so it's practical possibility rather than permission.