Basically the keystore is a 'safer' way to use your private key, since any thief would require both your keystore file and your password, instead of just your private key - so you're more protected against key logging or copy/paste hijacking and similar threats. You could just use the private key if that's what you prefer, it will have exactly the same end effect. If you are using this wallet frequently on an always online PC, then more security is always advisable. On the other hand if you are using it infrequently on a mostly offline PC, you may be comfortable just using the private key.