Also, when they edit old shows for syndication, they often restructure the commercial breaks to fall somewhere other than where originally intended. Done poorly, this sometimes has breaks falling in the middle of a scene. (Cozi edits of Adam-12, I’m talking about you!)
And in the case of a shows, like Doctor Who, which were not designed for commercial breaks, it can sometimes feel like an awkward cut. Thankfully they don't edit scenes out of Doctor Who to make them fit, they will show the entire episode. At least in Canada.