Google rewrites titles and meta descriptions when they think it makes sense. In theory, "makes sense" means they believe they have found a title that matches the user's search better.
Again in theory, matching the user's search better increases the click through and increases traffic to your site. Since we now live in an era when a preponderance of referral data is withheld, you have no idea of which searches are sending traffic to which pages. You also don't know whether Google is rewriting the titles for just some subset of users or for everyone (as far as I've seen, it's everyone doing the same search, but that could be wrong).
If you put all that together, you have no idea whether or not Google's title rewriting is helping you or hurting you (unless someone has cracked this, but I don't believe anyone has).