The Seekable option enables the seek bar for a recorded file so you can move forward or backward through the file. However, there are instances when a file cannot be made seekable during recording even if this option is enabled. In this case, you should use the Reprocess option with the Seekable flag enabled.
The opposite is when you record a file that was seekable but you want to make it non-seekable. This may force your audience to see the entire clip. In this case, you can Reprocess the file with the seekable flag off.