Sigil scratchpad
 

In the user folder there is a folder titled sigil -. scratchpad. It 30 Mbytes in size and contains 610 files.

Is this an automatic maintenance folder or should it be cleaned periodically? What would happen if these files were deleted?

Sigil used to create temporary files in that folder and remove them on exit. But if it crashed, the files would remain. So you can delete that folder now, if Sigil is not running.

Recent versions don't use that folder at all.