I touch on this at the end of the post. It's a short 15m 'discovery call', not a sales call. It's essentially a formality to intro each other, make sure we're human, and move onto email for any further discussion. Essentially, not all enterprises will shoot you a cold email to start the conversation, so this call is to capture those leads, with the end-goal of having all real discussion in email.
tl;dr: some enterprises will bounce if they don't see a 'book a call' button.