Consider your private key as your debit card pin code, your public key as your bank account no. which you usually give others to receive money, blockchain ledger as your trusted bank. Practically if you want to spend your fiat money that is stored in the bank you actually swipe the card and insert your pin code for the transaction to get processed successfully. Same is with cryptocurrency if you want to spend your coin/token that is stored in blockchain you actually need to insert your private key to get your transaction processed which proves you are the actual owner of your public address.