I think it depends. If you like the genre and the way the story is presented, they can be great. But it's subjective. Different people have different tastes when it comes to manga books.
Secretary Kim manga is popular for several reasons. Firstly, the art style is visually appealing. Secondly, the storylines are unique and full of surprises. Also, the character development is deep and makes you care about the characters.
The king might need a secretary in manga for several reasons. One could be to organize and schedule his busy schedule. Another could be to assist in communicating with various officials and subjects. Also, the secretary could help the king keep track of important matters and make sure nothing slips through the cracks.
The combination of workplace drama and romance usually grabs people's attention. Also, the character development and their evolving relationship can be quite captivating.