Advantages: you can boot from this and reinstall Lion and/or recover from Time Machine without needing an internet connection. There are a number of similar options that can be a little confusing, so here's an attempt at clarifying their differences: Then, after you replace the HD, you can boot from that and use it to reinstall Lion and/or recover your system from Time Machine.
I'll add yet another option to the list: before you remove the old HD, use the Lion Recovery Disk Assistant to copy its recovery partition to a flash drive.
