A blockchain is an immutable, tamper-proof record of events recorded by a group of computers called miners, and in addition to miners, who mine and submit blocks containing transactions, full nodes are nodes who keep and share a copy of a blockchain, and in combination with miners, make a cryptocurrency network ‘alive’.
Essentially, a full node is a replicator that contributes to the security of the availability of the data of the blockchain. They are complicated though largely self-maintaining, providing good, fast network service for blockchains is essential to keeping the network up and running.
Importance during early adoption
In the early stages of adoption of a cryptocurrency, there can be times where all nodes drop off the network and the network can be said to be ‘dead’. Miners need a full node to mine, but there must be at least two full nodes to form a network and three ensures it continues even if one node is disconnected or fails.
For providing services using the cryptocurrency
Full nodes are the backbone of cryptocurrency-based services, in combination with wallet servers. If you want to work with a cryptocurrency to receive payments in a cryptocurrency one option you have is having your own wallet to do this.
It is better to keep wallets yourself preferably at your own site and only for daily transfer needs but a full node provides access to the entire record of transactions, essential for preparing many kinds of transactions and monitoring the network for transactions relating to a set of addresses.
It is possible with some cryptocurrencies to have a wallet and full node separate, and with these, you can keep the hot wallet in separate infrastructure without needing to worry about storage space or ensuring availability, and this service is outsourced.