The world around us is very complex. To reduce its complexity people use models: they look at a part of the world and create a description how it behaves. To further reduce the complexity of these processes ontology modeling can be used. Ontology has a number of meanings, for example it can be a philosophical study. But it can be a practical tool, which can be called applied ontology.
An ontology provides a means to describe models in a consistent way. An ontology usually has principles to formally name categories, properties, concepts, entities and their relations. So one of the results of creating the ontology is a dictionary of terms used in the related domain. Ontologies are used in information science to represent data and knowledge, they can be considered practical ontologies. Also every field of knowledge creates an ontology to use inside this field, it can be done explicitly or implicitly.
I should also mention upper ontologies, which are used to construct domain ontologies. They provide the same principles and categories as domain ontologies, but these are suitable for more general use. As a result if several domain ontologies are constructed using the same upper ontology, specialists from different domains can better understand each other and better transfer knowledge between domains.
When we have an ontology we can build models more easily because we have more means and tools. And we the domain ontology we can create models more easily. It also means that we can create theories, hypotheses, and arguments more easily, because they are models of reality or they are closely related to such models. If people have knowledge about ontology modeling they probably can better understand other models. If people know a specific domain ontology it would help them to understand models in this field.
You may think that if you don’t know about ontologies you don’t have to use them. But in fact you most likely have used them already. If you are a specialist in some field you should know specific terms used in this field. And a dictionary of domain-specific terms is a product of ontology engineering and it is a part of the domain ontology.
So if you are a programmer you work with ontologies of a specific programming language and related concepts like programming in general. If you are doctor you work with the ontology of medicine and more specific ontologies of different sub-fields of medicine. And so on. Also some information science subfields are closely related to ontology engineering, for example a database design creates an ontology.