"All projects should show a progression from initial ideas to final outcomes." The dependence on good planning is essential if a good game is to be made in my opinion. Planning requires forward thinking and good organisational skills. Concepting is a tool used during the planning stage to help people visualise the idea you are wanting to work towards. it is an extremely useful tool that helps a teams' mind work in sync, and is also a useful tool to use when working on personal projects, helping you to finalise ideas that you might have had, giving you a platform to go on and create further more interesting ideas.
Planning comes in many different ways for different ideas or parts of a game. Planning a cutscene for example could be done through the use of storyboards, rough sketches that outline how the scene will plan out, a very cheap and time effiecient way of planning a cutscene through.
A storyboard by Cecil Kim for the God of War series
Another very helpful way of planning for idea generation amungst other things is using moodboards. Moodboards or "brain farts" as i like to call them are basically where you collect a bunch of pictures/bits of writing/colours or pretty much anything and bring them together. Its useful for when you need to help concrete an idea or even serves pretty well as a rudimentary piece of research.
This type of planning is often used by fashion students and profesisonals.
Personally, i find that i'm pretty bad at planning ahead and it's quite obvious from my work. I've found that alot of times i make concept art for the purpose of looking pretty rather than for helping me persue the idea into a more detailed and well thought through final outcome. However unlike me games companies would simply fail without great planning and concepting teams, one example of great concepting within games would be Guild Wars 2 by ArenaNet.
ArenaNet used the concept art made by their amazing team of artists to help promote the game and ultimately helping the game Guild Wars 2 become a major success. They blast open the doors of the MMO scene and took the world by storm during release. The art style of Kekai Ketaki really pushing the game forward.
The concept work is not just used for the planning stage of the game but was also used to help tell stories within the game through animated versions of the concept art.
some of the works used to help promote the game, they were even used in the game for loading screens.