Rules are one thing that all GMs need to set up their scenarii, their game sessions and in the last to “frame” the core settings they gonna use to lead their players.
Lots of the times, in addition of the core book for any RPG, authors and publishers like to release campaigns, which expand and maintain their RPG in the loop of the gaming world where almost every month, a new thing is on the loose.
As a GM and player, for some reason i barely bought any campaign, just because i am one of those who loves improvisation during game sessions, for which my scenario probably holds within two phrases. I give the plot to my players as an introduction and i enjoy to see what they can do out of it because i consider my players as the true engine of any game session, and also because i love to see players managing their characters, making them evolve.
To my point of view, we tend wrongly to believe that the one behind the screen is the master of the game. But if you consider all the aspects, i would tend to say the players are the true engine of every game sessions. They follow, they surprise you, they fight IRL for their character, they are either happy or disapointed.
Improvisation, to me, is the key of a success of a scenario or a campaign, because it brings colors, laughs, texture to your sessions and also brings to the GM some new leads, new expectations and make him or her adapt to the players for the beauty of the game !
But some GMs don’t like very much to go off road when they are leading a campaign or a scenario they wrote, either because they don’t enjoy changing their plan or because they are not comfortable to do so. Impro is like taste, some likes and others do not.
To the least i would say that as long as everyone around the table is on the same page, the fun begins !