chazzan prayer leader - Why do some people say the end of the 2nd bracha before Shema with the Shat"z?

I have noticed in many shuls (mainly Nusach Ashkenaz, though, I have seen in Nusach Sefard, as well) I have attended that some people conclude the end of the 2nd bracha before the Shema (Shacharit - ends with הבוחר בעמו ישראל באהבה ; Ma'ariv ends with אוהב עמו ישראל ) together with the Shat"z.

I guess this is a minhag? What is the origin / reason for doing this as opposed to concluding the bracha before the chazzan? Is there a problem with pausing between the end of the bracha and the beginning of Shema?