Why Packet radio (AX.25) frames are built like an envelope around the data: the header carries the addressing and control fields, meaning the source and destination callsigns, any digipeater path, and the control/protocol bytes that tell the receiving station how to handle the frame. The payload follows, and the frame closes with a checksum (FCS) and flag. So the routing and handling information all lives in the header.
Watch out The preamble is just the sync pattern or flag that lets the receiver lock on to the start of the frame, and the trailer holds the error-check sequence and closing flag, not addresses.
Like a letter: the header is the address on the envelope, the trailer is just the seal.