"If you want someone to marry you, why don't you just tell me that the first time you meet them?"

Same reasons apply here...

- I want to give you time to see bits of the course and learn about whether it's a good fit for you before making a (nontrivial for some people) purchase

- I want to make sure you're the right fit for the course too

- You're more likely to purchase after getting to know me and the course content, understanding the value, and overcoming your objections

Asking for your email address is the happy middle ground of "can I buy you a drink" between "saying nothing" and "asking someone to marry you".