Why do managers conduct interviews for the sake of it?

How do you, HR managers feel when you go through a whole process of interviewing applicants yet you already have someone for the placement. Many times job adverts are put out, people get interviewed yet a company already has some one they need but are simply interviewing as a formality. Gina

Dear Gina,
Companies may be required to advertise for a position and get both internal and external responses. So if the internal staff outshines all the applicants he/she will surely be considered for the position. On the contrary, this may be interpreted as if the position was already filled and the interview a formality.
However, some companies may advertise just to satisfy a policy or they might be seeking advice and ideas to handle projects hence calling a dummy interview to get insights from a few qualified applicants.
This is dishonest and unethical, but it happens some times.
As a job seeker, you should be wary if the interviewer is more interested in listening as they ask specific questions and avoid interrupting you at all costs yet provide no information on the job you are applying for. They take weeks without contacting you, then all of a sudden ask you to write something new, research a topic or request for another meeting.
Be cautious if the interviewers are extremely friendly and warm with you in every interaction, treating you as though you already work for the company. Finally, they stop contacting you and act as though your “interview process” never happened.
Nonetheless consider the interview process as an opportunity to learn whether you get the job or not.