This may not be a recent trend, though I've only personally noticed it in the last couple of years. Some shows will have a voiceover on the opening credits where the general premise of the show is explained, often - but not always - by one of the main characters.
- Arrow. Voice over by Oliver Queen, explaining his history and how he became The Arrow.
- Flash. Voiceover by Barry Allen, explaining who he is/how he became The Flash.
- Grimm. Voiceover by an unknown person (not a character of the show) explaining the general premise.
- Doctor Who. Only for a single season, but a voice over by Amy Pond explaining her history with The Doctor during Matt Smith's second (I think) season.
Arrow and Flash are broadcast on the same network, and have a lot of crossover, so it's not surprising that they'd both take the same approach. I'm not entirely sure which network broadcasts Grimm. However, Doctor Who is a BBC show, so it's not limited to a single broadcast network, or even one country.
What reasons are there for choosing this style of opening credits? Note that I'm not looking for subjective opinions/reasoning but verifiable information, such as comments from the people involved with those shows on why they made that choice, or the results of research that indicates it makes a difference to viewer retention, attracting new viewers partway through a shows run, etc.