Currently, you only have the option to "Render Previews" of the email you're creating from the Campaign builder. This should also be available from the Email builder itself. Many times, you discover issues in the email that you need to fix after rendering the previews, but you can only render the preview from the campaign builder. So to fix the issue, you need to go back into the content builder, fix the problem, and then go back into your campaign, and repeat as many times as it takes to solve the issues.
To compound this problem, if you are building an email that is not intended to be sent out as a campaign, but will only be used in a Journey, you don't ever get the option to render the previews. In order to see the previews in this case, you need to start building a campaign that you will never use.
You should be able to Render Previews and send a test message to yourself directly from the email content builder.
To add to this, being able to render the email based on subscribers in a list or simply being able to send a test email to a few different emails are standard features.