Your IT department wants to know why they need to create separate subdomains for Marketo email tracking links and Marketo landing pages, and cannot simply use the web domain you already have.
Each web domain can only point to one resource. Your company's web domain (www.example.com) points to the server or web hosting service that handles your company's web page. It cannot also point to Marketo's email link tracking server or Marketo's landing page web server. Additionally, since the link tracking server is separate from the landing page server, you need two different CNAME sub-domains, one for the email links (email.example.com) and one for your landing pages (lp.example.com).