SBV is understood mainly by YouTube and a few caption tools. CSV is plain text that every spreadsheet, database and script can read. SBV is at home in uploading and downloading YouTube captions; CSV is the usual choice for moving tabular data between systems.
The text is read as UTF-8 or UTF-16, whichever the file is. An older file in another code page is read using the charset it declares, or as Windows-1252 if it declares none, so accented letters in an unlabelled legacy file can come out wrong. A file that is not text, such as the picture-based subtitles of a DVD, is refused instead of being converted to nonsense. The result has the columns index, start, end and text, with times as HH:MM:SS.mmm, and starts with a byte-order mark so Excel reads accented letters correctly.