Project portfolios are helpful for grouping work and managing projects individually and as a whole. Many companies start out not utilizing project portfolios. Then, as the company grows projects become unmanageable and the need for project portfolio management becomes clearer. Portfolio management helps companies maintain efficiency and breaks project work load into manageable pieces. It is vital that whatever project management software you utilize, be sure it is scalable to include project portfolios. It is likely you will need it! There is way too much to cover in this short blog. However, a pretty good white paper can be found from Mosaic Project Services at the link below.
http://www.mosaicprojects.com.au/WhitePapers/wp1017_portfolios.pdf