Time & Material is a model of financial interaction between the customer and the developer. Literally, the materials were created while working on the project. The cost of work with such pricing is calculated not by the final result, but by the hours of work on the project.
This model is suitable for projects to develop customized solutions. These are the cases when the client asks for something from a series of "modern and convenient mobile applications", or when the product is innovative, a know-how idea.