Transfer learning does exactly as the name says. The idea is to transfer something learned from one task and apply it to another. Why? Practically speaking, training entire models from scratch every time is inefficient, and its success depends on many factors. Another important reason is that for certain applications, the datasets that are publicly available are not big enough to train a deep architecture like AlexNet or ResNet without over-fitting, which means failing to generalize. Example applications could be online learning from a few examples given by the user or fine-grained classification, where the variation between the classes is minimal.
A very interesting observation is that final layers can be used to work on different tasks, given that you freeze all the rest, whether it be detection or classification, end up having weights that look very similar...
