The end-result of good looking LaTeX documents, without (most of [1]) the pain.
I recently adapted a colleague's document from LaTeX and it was simply easier to redraw a slightly complex table in LyX than to understand the table markup. I think he'd be disheartened to learn how fast it was to produce the table.
Writing math is also a no-brainer compared to LaTeX. Whenever my LaTeX-using colleagues see me writing stuff in LyX, and the screen is filled with semantics rules that basically look just like they'll look in the final PDF, they go "yeah, I really have to check out LyX some time..."
Mostly they end up never checking out LyX and sticking with LaTeX, to their loss, but I've managed to convert a few people over!
([1] the remaining pain is related to dealing with conference and journal style files, which often have quirks in LaTeX, and adding LyX on top sometimes demands an extra layer of adaptation.)