9.1.1 Safe Methods
Implementors should be aware that the software represents the user in their interactions over the Internet, and should be careful to allow the user to be aware of any actions they might take which may have an unexpected significance to themselves or others.
In particular, the convention has been established that the GET and HEAD methods SHOULD NOT have the significance of taking an action other than retrieval. These methods ought to be considered "safe". This allows user agents to represent other methods, such as POST, PUT and DELETE, in a special way, so that the user is made aware of the fact that a possibly unsafe action is being requested.
Naturally, it is not possible to ensure that the server does not generate side-effects as a result of performing a GET request; in fact, some dynamic resources consider that a feature. The important distinction here is that the user did not request the side-effects, so therefore cannot be held accountable for them.
This matches quite nicely with my intuition about how the web works (at least the normal web). That might seem like something trivial, but in fact it is something profound.
Unfortunately, many, many web services violate the principle of safe operations. For example, they frequently have all interactions occur via GET, and use query parameters or headers or other conventions to stipulate whether the result is a resource retrieval, creation, update, or delete. This is (unfortunately) only one example of aberrant service design.
At first, this might not seem like such a problem. But as you start to think about consuming such services or providing support for services you've built, you realize that the conventions of HTTP and the architecture of the web represent a sort of "lingua franca". Rather than making your services do more, you are making your services easier for (potential) consumers of your service to understand and use. If you adhere to the architecture of the web, your services don't do any more than they would have done otherwise. Rather (and this is crucial) you have implemented your services in such a way that users of your services can very easily and rapidly understand their features, capabilities, and limitations. That is a huge advantage in today's fast-moving technology world.