My first two published novels didn't have chapter titles, but my third did. I don't remember when I started using them because things don't necessarily get published in the order I started writing them. For example, my current WIP uses them, but I had started it before I wrote and published Bad Fiction which also uses chapter titles.
I use them on most of my stuff now. I can't say whether I will always use chapter titles from now on, but I prefer using them over not using them. As Jeff Tanyard mentioned, it makes them look more interesting than a list of chapter numbers.
Plus I always put my Table of Contents in the front of the eBook, because that's where it would be in a printed book, so it ends up showing up on the "Look Inside!" I think it can perhaps help make the reader more curious about the book if the chapter titles arouse curiosity (without revealing spoilers). So, in that regard, it's an extra tool to encourage people to buy.