Why does a stream have multiple videos?

Requirement: Get a bunch of frames from a video file according to one frame per second. So I wrote the following function

import av
from numpy import ndarray
from loguru import logger
from typing import Generator

video_filename = 'Movies/123.mp4'


def frame_extract_pyav(video_filename: str, is_multi_thread_decode=True) -> Generator[ndarray, None, None]:
    with av.open(video_filename, metadata_encoding='utf-8', metadata_errors='ignore') as container:
        try:
            video = container.streams.video[0]
            video.thread_type = "AUTO"

            average_fps: int = round(video.average_rate)

            for index, frame in enumerate(container.decode(video)):
                if index % average_fps == 0:
                    yield frame.to_rgb().to_ndarray()
        except Exception as error:
            logger.warning(error)

What I don't understand is why is the type of container.streams a tuple? Why can a streams have more than one video?

Why is it the case that a streams can have more than one video? Does it have anything to do with the video encoding format? For example, is it possible for a common mp4 to have one streams containing multiple videos?