One valid reason is that its cheap. mostly even free.

The other reason that it is Open.. which means that if you need to change things to better suite your needs, you can do so, contrary to Applications like SAP or Microsoft products.

Website Content Management Systems

For instance you could setup your website and have it build in Dreamwaver of even Frontpage, but a mucht better solution would be to use Joomla, Drupal or WordPress.

You can simply download the software for free and install it on you domain.
Some hosting companies like MidPhase provide you with a one-click-installation for these content managment systems.

Once you have done that, change the look of your website (or have it changed) and start building on-line.
Manage your site from where ever you want, you just need a PC (or Mac) and an Interet connection. You can even changes that special discount page from your vacation address if you need to!

Office Programs

On of the best know alternatives for Microsoft Office is Open Office, which give you the same possibilities, but for free if you want it.
And if you want support, check out the forums, or buy support from Sun for a much better price than Microsoft will ever do.

Another lesser know Free office package is IBM Lotus Symphony which contains an Word Processog, Presentation software and a Spreadsheet program.
On of the best features in this program for me is the fact that you can create pdf documents directly form the software itself.

No need to install a seperate program of printer drivers, and the outcome is great

If you still want to go for Microsoft, I would suggest that you go for the Microsoft Office Professional 2007 version because that also contains the Microsoft database software that can help you setup some database to hold your contacts and other informational database you might want to use.
The standard database applications provided with the professional version will pay itself compared to the price difference for the standard version.