A common question that seems to arise when people are architecting their SharePoint Intranet is "Why should we use more than one collection?". "One site collection should be more than enough!"
Small Corporate Organisation
The simplest example I can usually provide is a small corporate environment of approximately 20 to 200 employees where the intranet site structure could be implemented using a single site collection. The Divisional sites and Team Sites would be implemented as sub sites within the initial intranet site collection. Due to the nature of a small corporate environment there would be one Site Collection Administrator who would be responsible for the overall site structure and perhaps a number of Site Members who are responsible for maintaining the content for the Divisional sites and various Team Sites.
If you choose to use a single site collection for your corporate intranet then you will need to be careful as you create "SharePoint Security Groups", templates, content types and site columns then your site may start to become rather cluttered. This may not initially concern you because "I'm the only person who needs to administer the site collection", but as the site grows and you start recruiting other people to assist you with the site collection administration then the clutter may begin to cause confusion with the administration of the site collection.
If you decide to move content to other site collections at a later date then you will find site dependencies such as content types and site columns will complicate the moving of content from one collection to another.
If you think you might need more than one site collection then it might be a good idea to start with multiple site collections from day one. An easy way to understand if you will need multiple site collection will be identified during the planning phase as you identify the audiences for content and what content types you will require.