I don't work for them so I don't know @joeyanoldvn why but I presume it's the same reason Steemit asks for your email/phone when you sign up. Instead of investing, you are signing up for their mailing list. I'm guessing you can probably use a rouge address is you're uncomfortable giving your real one. Perhaps they intend one day to have a governmentally accountable trading system like Coinbase. I'm sure you can get in touch with them and ask.