"I go to the dentist because I have a toothache."
I was confused, too!
Like から (kara), ので (node) indicates cause or reason. While から (kara) subjectively presents a cause or a reason, ので (node) highlights a cause-and-effect relationship as a natural course of events.
As the use of ので (node) softens the view of the speaker and sounds more polite, it's often used when you are gently expressing a reason, when asking for permission or when making an excuse.
財布を忘れてしまったのでお金を貸してください。 saifu wo wasurete shimatta node okane wo kashite kudasai 財布を忘れてしまったからお金を貸してください。 saifu wo wasurete shimatta kara okane wo kashite kudasai
Meaning: Please lend me some money as I forgot my wallet.
In this example, you are borrowing money from someone so you have to be polite. ので (node) will be a better choice here.
Hope this helps! :)
I think this is a really good exercise because it shows a couple other uses for the は character.