Site collections allow for your intranet to scale as well as provide for a model where it is easier to delegate control to power users for specific subsets of your portal.
Most people will tell you that you will need to have multiple site collections because of the database size restrictions but this is just a myth. There is no physical restriction on your database size, there's only the constraint of how long your backup/restore might take. For some background info - How large for a single SharePoint database as well as Tips on site collection sizing and How many databases for my x TBs of data? and SharePoint deployment capacity & performance planning 2003 & 2007 - what you need to know
There are however some other reasons for using multiple site collections - one of the main reasons being a controllable management of security. Security groups are shared at the site collection level so you will have one specific group of owners,members and readers for each division. When you use multiple site collections it will be easier to delegate security settings to the site collection administrator.
Some more background information: