Witnesses on documents are names that should not be ignored.
However, one should not assume that witnesses are always relatives. Sometimes witnesses are other people were also in the office of the person who created the document that your ancestor signed. Witnesses who appear on multiple documents of your ancestor are more likely to have had an association or affiliation with your ancestor–potentially through biology or marriage.
Of course that witness who appears only once could still be a relative and witnesses who have the same last name as your ancestor (or their spouse) should definitely be investigated.
Just don’t assume that a witness has to be related. They don’t.