TCP establishes a connection with a three-way handshake, numbers every byte, acknowledges receipt, retransmits losses and adjusts its sending rate to congestion. Applications get a reliable, ordered byte stream. MQTT, HTTP, WebSocket and TLS all assume it.
The guarantees cost memory for buffers and connection state, extra round trips before the first byte of data and retransmission behavior that interacts badly with radios that sleep or lose coverage for minutes: a stalled connection may need to be torn down and rebuilt, including a new TLS handshake. That is why constrained protocols such as CoAP and MQTT-SN choose UDP and add only the reliability they need, and why cellular IoT modules expose features like connection persistence across sleep. Where the device has a stable link and enough resources, the simplicity TCP gives the application usually wins.
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