What are .meta files for?

Did a search but didn't find satisfying answers...

It appeared after I downloaded a youtube video after choosing the High Quality option they now have on youtube. Anyone know what it does? I deleted it and it didn't seem to affect the file (the downloaded video).

Metafiles hold all the data that defines the content of the file. e.g. for a picture, you will find date taken, dimensions, pixels, etc. Have a look into Properties of a picture under the "Details tab". That's the kind of info stored in the metadata.
http://en.wikipedia.org/wiki/Metadata

Well considering you can find these things out in the file's properties, is there any point keeping the metafile?

If there were no metadata, you would have all blanks in Properties - where do you think the info in Properties comes from?

No I'm saying, maybe the metafile that is saved along with the downloaded video, contains this information but does not necessarily mean that if you delete the metafile that downloaded to your computer, as in send in recycle bin then delete, will all the info in the properties for the video file, be gone. I can see all the info now in its properties and the meta file has been deleted. Therefore I conclude the meta file that DOWNLOADED with the video file is kinda useless. I can still see everything in the video's properties.