Exception handling in drupal : How one can deal with warnings in Drupal?
The purpose of warnings are to warn you that something is wrong with the code you've executed. They are provided by PHP, not by Drupal (though Drupal can trigger them if required). Notices are similar, but considered less urgent. Warning/notices are not fatal, so don't halt the scripts' execution.

The way to "deal with" them is to simply fix the code, which might mean fixing your code, reporting a bug to a core/contrib issue queue, digging through a request to make sure that the value of variables being passed around are what they're expected/required to be, or something else.

Some people might tell you just to disable warnings/notices so that they're not bothering you visually, but as in all walks of life, hiding a problem instead of fixing it is a bad idea.

There isn't a catch-all answer to fixing every possible type of warning. You need to debug the site, find out exactly what and where the problem is, and apply your skills as a developer to fix them.

I don't know what the policy is for contributing modules that containing warnings/notices. I would hope that they are, at least in an initial review, rejected if they cause such things, but that might not be a practical expectation.

Regarding how Drupal deals with them: the answer is that it doesn't, really. You can switch on and off the display of error messages through the admin UI, but Drupal can't, and doesn't, have any interest in fixing these errors, or helping you any further to narrow down what's causing them. It's just a PHP app at the end of the day.

Incidentally, Drupal does implement its own PHP error handler (_drupal_error_handler_real()), so that it can log errors and exceptions in the watchdog, and/or to the screen.