Internal recruiting, by which is meant that an opening is filled in a company by transferring (or promoting) an existing employee, has the advantage that you are getting someone who is already familiar with the company in which he or she will be working, and whose performance has already been well observed by the employer. The disadvantage is that you then have to recruit someone else to replace the person who is being transferred or promoted, and that could be just as difficult as filling the position that you chose to fill by means of internal recruitment.
External recruitment, the hiring of a new person who has not worked for the company before, has the advantage that it does not create any new vacancy to be filled, unlike internal recruitment. It has the disadvantage that you never really know what you are getting. Everyone claims to be qualified, but sometimes it turns out that they aren't. Or even if they are, they may turn out to have personality defects or personal problems which will interfere with their work. And new employees will need more training than existing employees who are already familiar with the company.