The video container of course tells you what codec the video is, but it can lie or be otherwise incorrect. FFMpeg will use what it says as the default, but you can override it if you'd like (or if you have multiple decoders for a given codec).

But yes, you're right. 99.9% of the time there's no need to specify input decoder library

FFmpeg can have any numbers of decoders/encoders sometimes you want to use a hardware decode to read and a hardware encoder to write (like the intel), so you can override the default decoders/encoders for each codec.