For all practical purposes

For all practical purposes is a approach towards the problem of incompleteness of every scientific theory and the usage of asymptotical approximations.
When a physicist makes an approximation - which can not be justified on rigorous grounds - he or she may attempt to justify it by saying the results obtained are good for all practical purposes, meaning they agree with experience and approximation errors cannot be detected in practical measurements.
These types of theories are incomplete or loosely-based theories that nevertheless have very high agreement with experiments and tend to be very useful for all practical purposes.