"Do you ever pray alone?"
Yes- almost exclusively. I have prayed the Rosary twice, with Leah, and we both happened to be praying at the same time on a couple occasions. But other than at Mass and RCIA, I always pray on my own. I make the distinction because we never pray alone; we have God there with us. So we are never truly alone at all, but most people don't realize that. However, it's aimed at kids, and probably doesn't expect them to take the difference into account.
"What is one good thing about knowing some prayers by heart?"
You have them quick at hand when you need them. At a moment's notice, you can chime in without having to reference anything. Also, if you're praying the Rosary, you can simply follow the beads, put the prayers on automatic, and spend your time meditating on the particular messages that it has to impart.
Leah brought up another point in her 'yay catholics' post about them being weapons of spiritual warfare; and in times of crisis, having the prayer memorized means you're much more familiar with it, as you would be any weapon, and that can make the difference between life and death.