A light client delegates the responsibilities of running a full node to a 3rd party. By running a light client, you are trusting that 3rd party to perform critical functions, such as:
- Providing accurate balances and transaction history for your wallet.
- Broadcasting your transactions to the rest of the network.
Some reasons why you might want to run your own full node instead of using a light client:
- You don't want to (or can't) trust any 3rd party not to censor your transactions and/or show you false balances / transaction history.
- You want to take advantage of new/experimental features that aren't installed on available 3rd-party nodes.
- You want to protect your investment in IOTA by running your own node(s) to help secure the network (by validating transactions broadcast by other nodes).
- In order for your node to participate in the IOTA network, you still need to select "neighbors" (peers), so running your own node doesn't 100% remove the necessity to trust 3rd parties — but at least you can trust your gatekeeper.
- Any properly-implemented IOTA wallet (such as https://github.com/iotaledger/wallet) will never send your seed to the node under any circumstances.
- A node cannot alter a bundle/transaction without your seed. Even if you connect to a malicious node, the worst thing it can do is not broadcast your transactions, or to provide false information to your wallet — but it can't trick the Tangle!