The person you are corresponding with in Chat - is likely in another country. With that in mind, there is a mantra of reasons United would rather use this form and probably an accompanying authorization system - instead of allowing an agent overseas to collect your personal information.

One of those reasons could be PCI Compliance, Chat Archives for Quality Assurance (can't have credit card #s), or... concerns about personal information with these outsourced agents. There are really too many reasons to list.

If I had to bet on it. I have to imagine, you fill out the form. The agent types in an amount, or... selects a corresponding ticket/transaction, and then the amount is immediately authorized like a manual sale. This prevents the agent from seeing any corresponding card information.