Have you seen some of the Intranets out there? If you haven't, let me summarize the appearance of standard an out of the box Intranet like SharePoint, Jive or Office 365: Meh. Meh is okay. Meh isn't terrible or great. It just is.
Often times businesses use an Intranet to implement a new process or improve productivity using automation. The tool gains traction because form follows function. From a practical perspective, focusing on function first without worry about form allows organizations to focus on the bottom line and ROI as directly as possible.
The problem is, not everyone sees the world that way. In fact, a strong design aesthetic has produced global enterprises like Apple and BMW. Employees have higher expectations now than ever before when it comes to user interface design. Marketing and communication teams want to ensure a deliberate and concise message is communicated throughout the enterprise via the corporate brand.
I'm not saying all Intranet sites should be completely rebranded or redesigned from a UI perspective. But certainly the adoption and experience users have with your Intranet will be altered based on how they feel when they interact with it.
Takeaway: Leverage key stakeholder input to determine whether or not you want to invest in corporate branding your Intranet.