Finite element analysis (FEA) is used to predict how a model will react to forces in the real world. This analysis is a part of the product design cycle and examines the effects of forces such as heat, vibration and much more. FEA is carried out to optimize designs and understand their points of failure.
Creating a physical prototype to see how an object might react under real world stresses gets expensive. FEA allows you to digitally examine the stresses on the object and can let you run analysis on the digital prototype.
When the interiors and exteriors of an object are modeled engineers can easily spot points of vulnerability and run studies on the model to visualize stresses and course correct design flaws as they see them.
Boundary conditions such as point forces, distributed forces, thermal effects (such as temperature changes or applied heat energy), and positional constraints can be used as inputs in studies to see how the digital prototype responds to these conditions.
Calculations done by hand using a physical model could never match the accuracy of using software to model stresses; especially if the model is complex.