Bitcoin simplifies and secures the way you transact with other people, thanks to its peer to peer protocol. Each transaction is verified by several miners of the Bitcoin’s network, before being irreversible. The weakness of your own transactions could be the moment you send the transaction request: if you are using an infected PC, any malware or hacker could hijack your funds.
Using bitcoins can be compared with cash payments : a simple and convenient use, if you avoid pickpockets and pockets with holes... Ledger completes this lack of security : each transaction from a Ledger wallet requires an authentication provided by your device and/or the security card.