A full node is a program that fully validates transactions and blocks. Almost all full nodes also help the network by accepting transactions and blocks from other full nodes, validating those transactions and blocks, and then relaying them to further full nodes. 

By running a Bitcoin full node, you are supporting the Bitcoin network to become more decentralized and fostering the growth of Bitcoin.

Most full nodes serve lightweight clients by allowing them to transmit their transactions to the network and by notifying them when a transaction affects their wallet. If not enough nodes perform this function, clients won’t be able to connect through the peer-to-peer network—they’ll have to use centralized services instead.

Many people and organizations volunteer to run full nodes using spare computing and bandwidth resources—but more volunteers are needed to allow Bitcoin to continue to grow.

What is needed to run your own Full Node ? 

Minimum Requirements :

Bitcoin Core full nodes have certain requirements. If you try running a node on weak hardware, it may work—but you’ll likely spend more time dealing with issues. If you can meet the following requirements, you’ll have an easy-to-use node.

  • Desktop or laptop hardware running recent versions of Windows, Mac OS X, or Linux.
  • 200 gigabytes of free disk space, accessible at a minimum read/write speed of 100 MB/s.
  • 2 gigabytes of memory (RAM)
  • A broadband Internet connection with upload speeds of at least 400 kilobits (50 kilobytes) per second
  • An unmetered connection, a connection with high upload limits, or a connection you regularly monitor to ensure it doesn’t exceed its upload limits. It’s common for full nodes on high-speed connections to use 200 gigabytes upload or more a month. Download usage is around 20 gigabytes a month, plus around an additional 195 gigabytes the first time you start your node.
  • 6 hours a day that your full node can be left running. (You can do other things with your computer while running a full node.) More hours would be better, and best of all would be if you can run your node continuously.

Warning when Running Full-Node (Bitcoin Core) 

Several people have placed parts of known computer viruses in the Bitcoin block chain. This block chain data can’t infect your computer, but some anti-virus programs quarantine the data anyway, making it more difficult to run Bitcoin Core. This problem mostly affects computers running Windows.

Attack Target – Bitcoin Core powers the Bitcoin peer-to-peer network, so people who want to disrupt the network may attack Bitcoin Core users in ways that will affect other things you do with your computer, such as an attack that limits your available download bandwidth.

How to run IBD (Initial Block Download) :

https://bitcoin.org/en/full-node#what-is-a-full-node 

UPDATE : *Your node is not relevant for securing the network, but it is essential to verify that you received valid Bitcoin. Other wallets cannot prove that that. This is especially important during hard forks (?) .