The base installation of FreeBSD is meant to be lean and mean. It only contains the services absolutely necessary to operate the machine. Then, FreeBSD sysadmins are encouraged to install their own services, whether from the ports tree or elsewhere, choosing for themselves which software packages suit their needs.
For example, any webmaster will need a webserver but choosing which one depends on the needs. Some may need compatibility with older plug-ins from the Apache 1.x family while others will want the greater security and speed of the Apache 2.x family. Still others may need the highest speed and efficiency (but without CGI capability) found in the thttpd family. With FreeBSD it's easy, normal and even expected to pick and choose or even switch, with minimal pain, heartache or downtime.
Likewise, this holds true for FTP, DNS, mail, ssh, X-Window components and almost anything else. Even those services which do have a default system installation (such as sendmail for mail services) are designed to be easily swapped out with any other service the system administrator may choose. For example sendmail can easily be replaced by qmail:
cd /usr/ports/mail/qmail && make install && make disable-sendmail && make install-qmail clean
Please be aware though, one must configure qmail after doing that so please don't be rash and blindly replace sendmail after reading this without being ready to learn a new system.
This is all in stark contrast not only with Microsoft Windows, in which sysadmins and users alike are tightly bound to whatever Microsoft provided in the way of system services, but with many GNU/Linux flavours too. For example, veteran Red Hat Linux admins are familiar with the notion that Apache 1.x "goes with" Red Hat 7, but Apache 2.x "goes with" Red Hat 8 and so on. It can be a rather daunting, canny mess to try changing from one to the other. What's more, upgrading a base Microsoft or Linux system is likely to blindly change a system's services like it or not and meanwhile, upgrading the OS can make needed (and maybe mission-critical) services inoperable and comes with widely known risks, downtime and often, unhappiness.
FreeBSD mostly succeeds in drawing a much sharper line between the base system and optional services. It's easy to install whatever type of service is wanted from whichever vendor or group chosen, open source or not.
Moreover, FreeBSD is truly open and free. For highly specialized projects the source code itself can be studied and modified. While this step is not taken by most FreeBSD users, it's a comfort.