Security ... think about it!
Once you have created a volume, you can then mount it on your computer as a normal drive and use it just like you would use your C: or D: . The benefit you get here is just before the data is written to the mounted drive it is encrypted and likewise decrypted when data is read from it. When the drive is unmounted, the volume resides on your computer like a regular data file with some encrypted binary text that no one can understand. The volume container can be treated as a regular file and can be copied on USB or transferred to another computer where it can be mounted to a drive again using TrueCrypt, thereby giving you portability with your secured data.