What does TCP provide that UDP doesn't?

TCP turns the unreliable, packet-by-packet IP layer into a reliable, ordered byte stream between two endpoints, and it does so through several mechanisms UDP lacks: a connection established via the three-way handshake; sequence numbers on every byte plus acknowledgments from the receiver, so lost segments are detected and retransmitted; reordering of out-of-order arrivals and de-duplication; flow control so a fast sender doesn't overrun a slow receiver's buffer; and congestion control so senders back off when the network is congested. UDP provides essentially none of this, it's a thin wrapper over IP that sends independent datagrams with no connection, no acknowledgments, no retransmission, and no ordering, so datagrams can be lost, reordered, or duplicated and the application is never told. The price TCP pays for its guarantees is per-connection state and buffers, handshake round-trips before any data flows, and head-of-line blocking; UDP's price is that any reliability or ordering you need, you build yourself. So the distinction isn't "TCP good, UDP bad", it's reliability-with-overhead vs best-effort-with-minimal-overhead.