This question already has an answer here:
Why do we need to resample from an initial set of samples when using bootstrapping? Why don't we just take fresh sets of samples from the original distribution? What is the justification behind resampling? Or is it just a computational trick?
EDIT: I understand that it is usually very expensive to get fresh samples, but assuming we have access to a generator of the original distribution, does it still make sense to resample due to some theoretical reasons?