Bluetooth/BLE has the unique problem that you don't own the code on both sides of the wireless tx/rx. In webdev, you have control over client side and server side code and can make them work together in some unique way, without worrying about interoperability. But with Bluetooth/BLE devices, not only do you need to be able to communicate with random peers you've never seen before, those peers also need to be able to connect to countless other BT/BLE devices and read information about the services available on those devices. In order to enable this interoperability between BLE devices, a standardized format needed to be developed: GATT. GATT is the very highest layer of this interoperability and is considered "user-level" from the perspective of a BT firmware dev. In BT classic, the SDP table provides nearly the same service as GATT, except that data isn't carried over it, just service discovery.
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Bluetooth/BLE has the unique problem that you don't own the code on both sides of the wireless tx/rx. In webdev, you have control over client side and server side code and can make them work together in some unique way, without worrying about interoperability. But with Bluetooth/BLE devices, not only do you need to be able to communicate with random peers you've never seen before, those peers also need to be able to connect to countless other BT/BLE devices and read information about the services available on those devices. In order to enable this interoperability between BLE devices, a standardized format needed to be developed: GATT. GATT is the very highest layer of this interoperability and is considered "user-level" from the perspective of a BT firmware dev. In BT classic, the SDP table provides nearly the same service as GATT, except that data isn't carried over it, just service discovery.