By purchasing Sun, Oracle is elevated into the ranks of Dell, HP and IBM, giving them the ability to sell hardware as well as more fully exploit some important Sun software. It also puts them in a better position to compete with Microsoft by giving them control of both a major server operating system and some major business software begin run on it.
In their joint press release, Sun and Oracle cite "substantial long-term strategic customer advantages to Oracle owning two key Sun software assets: Java and Solaris." Sun's Solaris operating system is the most widely utilized platform for Oracle databases, which is Oracle's largest business sector. With the acquisition of Sun, Oracle can optimize the Oracle database for some of the unique, high-end features of Solaris.
There is some worry for proponents of the major open source projects that Sun heads up, like OpenSolaris, MySQL, VirtualBox, OpenOffice and Java itself. It will remain to be seen what Oracle intends to do with these projects and how, if at all, they'll integrated them into their product offerings.