This applies only if you want to self-host Khofly API.

Open Exchange Rates API is used to fetch the latest data on the server. To save on cost Khofly API periodically ( 1h ) fetches the latest rates and saves them on the server instead of client spamming OXR API every time the currency convert Instant Answer is used. Free plan gives you 1000 requests a month and if you do the math ( 1 req * 24h * 31 days = 744 reqs ) you won't be paying anything but you still need to subscribe and get your API key here.