Why modelVersion of pom.xml is necessary and always set to 4.0.0?

It is always set to 4.0.0 in Maven 2 and 3, because, at present, there is no other model.

Notice that modelVersion contains 4.0.0. That is currently the only supported POM version, and is always required. [source]

But it wouldn't necessarily need to always be set to 4.0.0 if there was another version of the model. A POM has to comply with a model. Let's say Maven 4 comes up with model 4.1. If you write your pom to comply with 4.1, it wouldn't be compatible with Maven 3 and model 4.0.0.

It's defined as a mandatory, possibly to enforce a specific XML model in case new models are defined.

model version is the version of project descriptor your POM conforms to. It needs to be included and is set. The value 4.0.0 just indicated that it is compatible Maven 3.

The Correct answer should be the combination of the answers by @Toumi and @Boj. Also have a look at https://cwiki.apache.org/confluence/display/MAVEN/POM+Model+Version+5.0.0 for more background of this.