Even Vitalik Buterin, the creator of Ethereum, does it: he recently sent 0.1 ETH before transferring the remaining 1499.9 ETH.
There are several reasons at once:
- The blockchain has no mechanism to undo a confirmed transaction. This includes cases where a payment was sent to an invalid or non-existent address, or where the transaction was signed with a private key that became known to others.
- Some exchanges and wallets have dynamic addresses. That is, the deposit address may change after a few weeks, so it is better to double-check everything before sending.
- Protection against phishing. Some viruses are able to spoof the address when you send cryptocurrency. As a result, your tokens will go to scammers.
- Human error. There are situations when you accidentally copied not the whole address, but only part of it. Or mixed up networks if you use several blockchains with different types of addresses.
- The exchange or the wallet may fail to process a deposit for technical reasons. This is impossible to predict and it is better to check everything with a test transaction.
- Insurance against malfunctioning crosschain bridges. Transferring tokens from one blockchain to another: a very complicated process. There can be problems with slippage, low liquidity, or various errors. If you are sending tokens over an unfamiliar bridge to another blockchain, it is better to be safe.
Lifehack: many wallets and exchanges support “white address” lists. You can save trusted addresses in bookmarks to simplify funds transfer and eliminate possible problems described above. It will also prevent you from withdrawing funds if your account is hacked.