Let's go through your question step by step.
Is that Houdini used for modelling ?
Basically yes but it depends on what you want to model. Houdini has strong and weak spots. Houdinis strength is on simple and/or repetitive models like a big pile of oil barrels or rocks or whatever. Houdini is also quite capable of models where there is no specific reference. A landscape for example or a forrest have no specific look, they could look any way and rarely look fake/unrealistic, for stuff like that Houdini gives you pretty handy tools. Whereas when you want to model something off of a specific reference, a car, a digital double of whatever hero element (human, animal, machine), Houdini quickly shows its weak spots because Houdini is not quite capable of/meant for precise, detailed models.
What is so special with rather than using Maya ?
The focus is what makes it special. Houdins focus is rigid body and fluid dynamics and particle simulations, with a few capabilities to model. Mayas focus is on modeling and animation, with a relatively few built in capabilities for rigid body and fluid dynamics. It's interesting to see that both are currently trying to improve their weak spots by adopting concepts from the opponent and thus they seem to converge somehow.
Is that Procedural Modelling is the only difference ?
Somehow yes and somehow no. Maya to some extend is also nodebased and procedural but Houdini is more technical and Maya rather artist orientated. In Houdini you have more control over your nodetree, changing things and have them update live but it also requires a decent amount of scripting/expressions and thus it's less intuitive.
That was a quick and compressed version of my point of view. Bottomline is it depends on your needs, what you want to do. When you're interestet rigid body and fluid dynamics Houdini is your choice. When you want to go for serious precise detailed modeling and texturing you'd want to choose Maya or Max.