I'm still lost where the backup can be utilized. Can the backup only sync to your own computer?
Yes, you can reload your backup in a fresh new install of Bitcoin-QT, it will rescan the wallet and find all your transactions.
Further, the only way for someone to access my wallet (not a backup) would be to get on my physical computer?
No. A malware in your computer could access it. If it's kept encrypted, the malware would only be able to steal your money if it also steals your password (assuming your password is strong, otherwise it could bruteforce it). Stealing a password is not really difficult for a malware though: it just need to wait for you to type it.
If you want to own a sensible amount of bitcoins (like, more than the cash you'd hold on your pocket while outside, for ex.), then you should search for more secured configurations, like offline wallets, paperwallets, USB-live systems etc. Or wait for
Trezor to come out.
The key to security is never to keep your private keys in the same computer you use for mundane tasks, like surfing the web. Even if you're a security expert, the computer you use to surf the web is always vulnerable to some degree.
