They are very helpful in many types of spellcasting for that reason, Sabata. Perhaps that's why they might be considered the traditional pet for witches, at least stereotypically. I know one reiki master who uses his cat as an assistant during healing sessions and since cats can see spirits they are capable of keeping the bad ones away. However, some cats have to be taught to not bother the good ones, else they become a bit of hindrance during necromancy.
Just thought I'd share to get the knowledge out there.