Every application and utility serves some purpose, but for every program created there is likely another (at least) that performs the same or similar task. For instance, there are a number of Web browsers, office suites, and text editors available in addition to multiple maintenance utilities, disk utilities, and file recovery suites. While having just one program that performs a specific function will suffice most of the time, if it stops working or if bugs in its code prevent it from working then you may find yourself limited.
I was recently contacted by MacFixIt reader "Terry" who was having troubles with iPhoto crashing when run after updating to OS X 10.6.6. Terry had run Disk Utility's permissions repair routine without any luck, and we were going to try some other troubleshooting options such as removing iPhoto's preferences file. Before doing this Terry decided to try running a permissions fix again, but this time using Drive Genius instead of Disk Utility.
Once the second permissions repair attempt was completed, iPhoto began working again, indicating Drive Genius was successful at repairing the permissions problem whereas Disk Utility was not.
While it may appear odd that one would work when the other did not for essentially the same task, perhaps this was due to slight differences in the procedures used by Drive Genius. In this case, having two utilities that were capable of performing a permissions fix was helpful.
If possible I recommend people double up on their tools and utilities instead of relying on only one to do the job. When troubleshooting and performing cleaning or fixing operations, doing the task with two or more utilities will not only provide additional confirmation of the problem, but also will check in multiple ways that it has been completed.
For the tools you do double up on, research them to see whether they are just interfaces for the same underlying process, or if they have their own unique approaches to the task at hand. For instance, many maintenance tools that check filesystems are just front ends for the built-in "fsck" command that Disk Utility runs to check and repair filesystems. Additionally, some tools that support antivirus use the free ClamAV engine and virus definitions, so if you have two of these then you will be essentially using the same underlying tool to scan for viruses.
This is not only true with utilities, but also with Web browsers and other office and productivity tools that you might use to open various Web pages and files on your system. Both Google Chrome and Safari use the WebKit engine to render HTML and other Web content. Being based on similar technologies they have a higher probability of sharing incompatibilities with various Web content, so if you decide to have multiple browsers on your system, be sure in addition to these two that you also have Firefox or Opera, which are based on different technologies.
Beyond Web pages, a last example that comes to mind is working with the numerous audio and video media files that are out there. While QuickTime will work in many cases, it lacks support for numerous codecs, or specific "wrappers" for media files. The freeware media player VLC will allow you to view much of what QuickTime will not, but even it may have trouble with some oddly encoded files. Therefore, having both it and MPlayer OSX Extended installed (which may play some files that VLC cannot, and vice versa) should be enough to allow you to view and manage most media you will come across.