Code can be modified to exactly match program requirements. With a commercial product, changes can't typically be made without incurring massive cost. So, users learn to 'get by', sacrificing functionality, or creating workarounds that ultimately can be become so complicated as to adversely affect productivity. Alternatively, the company that produced the software can be contracted to modify the software but again, at typically massive cost. A consulting company could also be hired to create a completely custom solution, but this is exactly the case where open source software is so beneficial: open source software provides a solid, well written, heavily tested, and well supported foundation from which to build the custom solution, saving time and money, and ultimately producing a better solution.
Real, direct support for open source software is available from the developers who wrote the code. Answers are often returned within hours, sometimes minutes of asking, if not from the developers, then from an informed community of users, users who have typically experienced the same issues and can suggest immediate solutions.
Open source software is reliable. As problems are discovered, they are fixed very quickly, for several reasons. First, pride of product. Unlike a large company where developers are nameless, often come and go very quickly, and really have no personal stake in the product they help produce, open source developers are personally accountable -- their reputations are at stake.
As well, because the development process is completely open, it falls under far greater scrutiny than that of a private company. The often heard excuse that "I know the code is full of bugs, but the sales guys needed the release out the next day" just doesn't happen with open source software.
Finally, many eyes see the code behind open source software, and therefore tend to find more bugs. Even more important is that those eyes are educated eyes who provide valuable feedback that can more quickly effect a solution. In some cases the feedback can be a fully tested correction for a discovered bug.
Open source software is most often produced by highly skilled developers who have worked on large projects, and understand the needs of the users within a particular domain. Further, as an open source package grows, users contribute back to the project, with supplemental code or with modifications that enhance the base functionality.
Open source software needn't solve the needs of everyone in a single package, and therefore, can provide a specific, customized, and finely tuned solution.