Hello there.
I am currently mainly learning by using korean podcasts and I have noticed, that although most of the sentences are formed using the 요-form (polite informal), the speakers tend to throw in a sentence using the 이다-form (polite formal) from time to time.
Is there any particular reason why native speakers usually do this? I googled but all I found was “you can use both”, which isn’t particularly helpful. My assumptions would be that it is either for hilighting important information or to mark when a topic or subtopic has been finished and the speaker wants to move to something else. I haven’t looked at this in detail, though, as I thought it might be easier to ask. 
Thanks in advance