It's worth thinking about UX in terms of an evolutionary approach to designing applications; and in my opinion, it's useful to think of product design as the field's real world counterpart.
Think about the design of any product, for example a plastic tub:
The product can be designed and built according to base fundamental requirements (e.g. needs a lid and a container)
... or alternatively it can be designed and built according to the researched needs of target users (e.g. as the tub will be containing hot liquids, and will be used in an environment where it might be accessible to children; it needs to be able to be able to be 'childproof', water-tight, and insulated so it can't burn those who touch it).
Both methods achieve the same aim, but the second methodology is
likely to create a more successful product, because development is
focused on achieving goals of real users.
While the example given is overly simplistic, the design of an application or website can be considered in exactly the same way.
In the early years of web and desktop application design, refinement of an application according to users' needs wasn't a priority. However as the industry has developed, so have the techniques and strategies available to practitioners.
Because the factors that need to be considered when designing a website or web-application are wide-ranging and interact in complex ways, a range of disciplines are useful aids to achieving a considered approach to design.
Each of these disciplines examines user requirements from a different angle and provides useful strategies and techniques for helping to create a software product that is based upon the goals and needs of real users.
When all of these disciplines are considered as a whole, they're conceptually referred to under the banner of UX (or User eXperience) because they all directly affect the way that your user experiences using an application.
It's very possible to create a software product without paying
attention to UX; but doing so is likely to provide a lower-quality
product that will ultimately create less value for your users. And
ultimately this reduction in value, potentially translates to less revenue for the business providing the software product.