Two options are available on the download menu.
Licensed downloads come with your license key built into the software. In most cases, this is the better option because you skip the input license key step
Public Release downloads begin in trial mode, and require a license key for full activation. On older versions, this is the only available option, and in some cases, you may need a download that does not have your license key attached. For those cases (e.g. deploying to a client's computer), this option is available.