An oracle request has many parameters. It is very common for requesters (e.g., a data feed) to make repeated requests with the exact same parameters. In such instances, it is wasteful to pass all of these parameters repeatedly.
Templates are on-chain records of request parameters that the requesters can refer to while making requests. Additional advantages are reducing boilerplate code required to make a request, improving UX and allowing large parameter payloads (e.g., off-chain computation specifications) at no additional gas cost.
Each template is identified by a templateId, which is the hash of its contents. This allows Airnode to fetch templates with a static call, and verify that the received parameters are not tampered with.