Just out of curiosity, if you haven't logged on your BTC client 3 months in and received a payment a week after you stopped using the client.
Wouldn't it be possible that if we didnt download the full blockchain the client's wallet wouldnt be updated properly?
To calculate wallet balances, the client just needs to know of any transactions where one of its addresses is an in/out of a transaction. In a client that downloads the entire blockchain, the balance is updated when the blockchain update reaches a relevant transaction. If the client relies on a server (like Electrum) then it would be easy for the server to send the right data when you open the wallet.
Ohh alright that makes a lot of sense now. I would assume the server downloads the blockchain and notifies the client of the transaction etc?