You have a recorded lecture or interview and you only want to listen. Turning it into an MP3 makes it small enough for a phone and easy to play on a walk.
Most online converters need you to upload the whole video first. A recorded meeting or a private interview is just the sort of file you should not be sending to an unknown server, and a large video can take longer to upload than to convert.
Doing it in your browser avoids both problems. The sound is read from the file on your device, encoded to MP3 there, and handed back to you without a single byte leaving.
If a video will not open, it is usually an older format such as AVI or WMV that your browser cannot play. Videos recorded on a phone or saved as MP4, MOV or WebM almost always work.
About one gigabyte, because the sound has to be held in memory while it is read.
No. Only the sound is used, and your original file is not touched.
128 kbps. Speech sounds clean at that setting and the file stays small.