Using a prototyping tool means being able to design a prototype of a web or mobile application without the need for programming skills. All the prototyping tools I know of function with a drag and drop technique. You can simply click on the predefined UI element and drag it into your screen. Working with prototyping tools to design wireframes of the product you are planning is helpful during the process of conceptualization. First of all, ideas can be visualized. Mostly the concept is then more specific than a written text for example as there is no way of avoiding certain aspects of the functionality. Secondly, the use of a prototyping tool enables good communication in your team. The hand-written design of the mockups that prototyping tools often offer motivates people to criticize or come up with new ideas as they feel the conceptualization is still in progress.
The wireframes can be easily changed (e.g. if you need to modify basic parts of your prototype). And thirdly, you can run usability tests in a very early stage. Working in a team, testing and involving other stakeholders in the conceptualization prevents you from making mistakes in the UI design and helps you to create a satisfying interface. These are all advantages of working with a prototyping tool. I am often surprised when people still think they don’t need a prototyping tool, because they can code. It’s not about your programming skills. Even a programmer wants to save time, doesn’t he? Besides, the prototype can be used as specification for developers. Well, in any case a prototyping tool saves time and money – no matter if you are an UX designer, a programmer or any other person involved in the project.