In the first chapter where locks were introduced, there was no mention of how locks work. It would be possible to implement locking in the database by just allowing one query access at a time irrespective of what kind of work will be done. However, this would be woefully inefficient.
Keeping to the analogy of a traffic light, another approach is grant access based on what work will be done. A traffic light grants access to the intersection not just to one car at a time but to all driving in the same direction. Similarly, in a database, you distinguish between shared (read) and ...
