It's pretty easy to just re-do your poster with an entirely different content and not have to worry much about layout. For somebody who may be uncomfortable with (expensive) GUIs for poster layout, which would force you to think about every little aspect of design and layout, a LaTeX template does a good job of just getting things done.
Why use vim+makefiles to build a big C/C++ project when you can just use VisualStudioC++ to do it?
LaTeX is a great way to make quality documents, really appealing, with very little investment beyond an afternoon or two of study - and, with sites like this, even less of an investment in LaTeX is required before something useful can be produced.
The beauty of this site is that it presents very tangible aesthetics to a design process that is not usually associated with such results, beyond the cognescenti. LaTeX can easily make appealing, beautiful documents, in the time it takes MS Word to boot its buttonstrips ..
If you've already written a paper in LaTeX, it's easy to copy and paste the equations, diagrams, bibliographies, etc. And they'll look good (especially equations, and also diagrams if you've used something like TikZ.)
It's really good if you want to include lots of "sciency stuff" like formulas, source code and maybe graphs. Also Bibliographies are easy in LaTeX. Plus it's free, and you don't have to learn a new tool.
I concede that I may be missing something though, since everyone else seems to like it.