Actually i want to know why these model(like exponential,weibull or..) founded??(I know first of all used in medical(to evaluate number birth or dead of some special people))
But for example in engineering or in computer science! why we need foe example exponential lifetime??
Have you google searched or used Wikipedia to read about how they were first surmised. Exponentiating has been around for awhile, while as you said Weibull can be in survival models (proportion of hazards regression).
The non-constant model is often more realistic.
The constant model is simpler.
In my rusty opinion.
Some scientists like to use a couple of models. You can see if there's drastic changes in results from a couple differences in assumptions. Then you get an idea if you need to be real careful in your assumptions. And helps you pick the final model.