The regular method used to delete files, is Recycle Bin, included in Windows. All deleted files are sent to Recycle Bin special folder, you may open the Recycle Bin and select what you want to restore (undelete).
This depends on Recycle Bin configuration, it will recycle deleted files to use only allowed disk space, etc. If you delete a file to Recycle Bin, the file is deleted even if have Read Only attribute.
Some functions of Comparator Fast allows you use a faster deletion method: Not using Recycle Bin. Using this method, files are not sent to Recycle Bin special folder, are permanently deleted. But faster deletion method is not able to delete protected files, like Read Only files. If you need to use faster mode with Read Only files, you need to remove the Read Only attribute first, or use the regular method: delete to Recycle Bin.
The faster method is usually enabled if you press and hold the Shift key while you answer Yes before proceed with deletions (at final confirmation).
Some options of Comparator Fast allows to force deletion of Read Only files when using faster deletion method (not using Recycle Bin): usually the force deletion of Read Only files is enabled if you press the Ctrl+Shift keys while you answer Yes before proceed with deletions (at final confirmation).