In case you are wondering why, take a minute to browse the SugarCRM download page. If you are unsure about which file to download, the Download Wizard is a useful tool that will guide you through the selection process.
If, however, you are just browsing around, you might note that some of the available choices are labeled with the tag "FastStack." These differ from the main install package--usually named SugarCE-6.0.1.zip or the like--in that they are intended to simplify the process of installing not only Sugar, but also the underlying technologies on which it relies, i.e. PHP, MySQL/MS-SQL and Apache.
If the idea is to simplify the process, why wouldn't that simply be the default or only install method?
Although it simplifies the process of installing the aforementioned technologies, the use of the FastStack installers can present a number of challenges in the short and long term.
As with any other software package, the FastStack installer can experience problems. Because it is intended to perform so many functions, pinpointing the exact source of one of its problem can be difficult. This is further compounded by the fact that the intended audience are users that perhaps are not comfortable with the intricacies of installing MySQL, Apache, etc. In short, it is not a good combination of factors.
You will also discover that the user community at large does not have a ton of experience with it, because the majority of users rely on other methods for installing their instances of SugarCRM. As a result, you will often be left on your own to troubleshoot potential issues.
Another matter that needs to be taken into consideration is that the collection of software it installs is treated as a single entity, at least to a degree. What this means is that you lose the ability to easily perform tasks such as upgrading the included version of PHP, unlike would be the case had you installed PHP independently.
This can be a problem down the road, specially if you wish to upgrade to a version of SugarCRM that requires a newer version of PHP or you need to upgrade PHP to apply a security fix or other important patch. In such scenarios, you are left with few feasible options beyond reinstalling each component independently.
However, the FastStack installers do have their use. As stated earlier, they simplify the process of installing the necessary components that SugarCRM requires. If you are new to the LAMP/WAMP (Linux/Windows-Apache-MySQL/MS-SQL-PHP) world, this will help you get up and running quickly.
You might also find good use of it for quickly deploying instances of SugarCRM that are used for development, training or evaluation purposes.