Could you explain why someone using OSX would need to worry about downloading Linux? If I were to be interested in using Linux I would grab a PC and stick it on to that. Linux is a great project and long may it live, but why would you put an open source project on your Mac if you were remotely worried about security?
This could be a very lengthy conversation.
Linux is an Operating system. Not just an application. It offers many robust features that are part of an extremely large open source community. Many Distributions of this Operating system are extremely secure, stable, and can offer extremely good performance. All for free. It is a fork of Un*x, similar to OSx (Based of BSD). But unlike OSx, Linux has remained true to the open source community.
Linux offers many compelling different windows managers. This is the "display" layer. Apple essentially locks you into their own GUI (Similarly so does Windows). Linux offers the ability to customize and change this to a multitude of different and highly customizable UIs'. From Gnome to KDE to even older XFCE. There are hundreds of different UI's
Being completely open source, the Linux community at large constantly updates it, preventing malicious code, bad code, and stagnation of features. Anyone who knows how to program is free to develop and submit by signing up for any of the different linux projects. This tends to mean that features come faster. Updates are done quicker, and are done more transparently than in the closed source world. Bug fixes and security enhancements can be almost instantaneous since there's little need to wait for the primary source (Apple or Microsoft) to distribute their own updates.
Hardware requirements are extremely low. Some linux variants can run on as much as 15-20 year old hardware. Got an older Mac that is struggling to run El Capitan or Yosemite? You will likely still be perfectly fine running some form of Linux. Because of the open source nature and distribution model, you will likely easily find a work around / support for older out of date hardware than Apple has long abandoned. Some older Macs for example stopped receiving updates around Lion when Apple made a hard rule decision not to support older EUFI's in newer OSx. They would have stopped receiving security udpates and patches shortly after. However, those users can still run a perfectly fine, up to date, and secured Linux.
Security measures can be better in Linux (though arguably harder). Linux is typically faster due to the open source nature to respond to security issues. It can also be locked down for internet facing servers more robustly.
Like earlier mentioned with it's older hardware support, it's capable of running on just about any hardware configuration. from Mac's, to PC's, to thin clients or embedded. If it's got a CPU, some form of Linux has likely been ported to it.
Linux out of the box supports multiple programming language support, including built in compilers and is a very proficient development platform.
Did I mention this is all available for free?
While no concrete numbers are likely to be found, Linux, is probably the biggest backbone OS of the entire internet. Aside from Windows back ends, most of everything else on the net, is likely running off a un*x or linux box, or windows.
you may never NEED to download linux, but many many people do.And at the end fo the day, A Mac is just another Personal Computer. if you have a spare mac laying around and want to install linux, why not.