One of the key reasons for this setup is to prioritize efficiency and security:
When browsing through various websites, you typically access them over secure SSL (HTTPS) connections. However, enabling SSL can be more expensive than using standard HTTP.
As a result, when simply navigating the website as a visitor, you are often on HTTP. It's only when you need to authenticate or perform sensitive actions that you are redirected to a separate window, which operates over SSL- allowing you to see the URL address (verifying the SSL connection, checking certificates, etc.).
This practice can also be observed in online shopping platforms, particularly during the checkout process where the transition from HTTP to HTTPS occurs.
While it may not be the most secure method (as using SSL throughout the entire session would be preferred), cost-effectiveness usually takes precedence, especially for smaller businesses.