What is the main job of a Bitcoin full node?

Detailed explanation

A Bitcoin full node checks blocks and transactions with its own software. During initial synchronization, it processes the blockchain from its known starting point to the current tip. Among other things, it checks that transactions spend existing UTXOs, satisfy spending conditions and create no bitcoin from nothing, and that blocks have valid proof of work and an allowed subsidy.

Many full nodes then relay valid blocks and transactions to other peers. A pruned node can later delete older block data and still be a full node if it previously validated the entire history and continues checking new data itself. Complete storage and complete validation are therefore not the same.

A full node does not automatically create blocks or hold other users' keys. Its key benefit is letting its operator verify the rules independently. It can reject invalid data even if a miner, exchange or other service calls them valid.