"Burning in" subtitles means that the subtitles become a fixed part of the video imagery. Unlike optional subtitles which can be switched on or off by viewers, burned-in ones are always displayed and cannot be removed.
You might choose to burn in subtitles for several reasons:
Platform Constraints: Not all platforms support detachable subtitle tracks. In such cases, burned-in subtitles become essential.
Uniform Message Delivery: Burning in guarantees that all viewers, regardless of device or settings, get the intended message without distractions.
Promotion & Marketing: It's especially favoured for trailers and adverts. Here, creators want a consistent viewer experience without the variable of adjustable settings.
In essence, if you want a standardised viewing experience and message delivery, especially on platforms with subtitle limitations or for promotional content, burning in subtitles is a practical choice.