Whatever the similarities, most mythical figures were not meant to be taken as actual, literal beings, existing in time and space, not even by devotees.
The most important quality Jesus possessed, in the eyes of his followers, was reality: he was an actual human being who lived at a particular point in history, in a specific place. That is one reason accounts of his life contain references to historical events taking place at the same time, like "In the fifteenth year of the reign of Tiberius Caesar—when Pontius Pilate was governor of Judea."
I think some similarities exist because mythology deals with issues that are most significant to human beings, in ways that resolve or explain them. Others are coincidental. I do not find them as overwhelmingly important as some people do, because I don't think the similarities tell us all that much, and because there are always people who will find similarities if they look for them. Two equally intelligent people can look at a series of myths or mythic figures, and one of them can conclude, "It's all the same!" and the other conclude, "They are so completely different!" Both may be right; it is a very subjective thing.
I think the finding of elements of other mythic figures in Jesus, or of other belief systems in Christianity, is often done in a rather arbitrary way. Finding elements of Jesus/Christianity in Paganism is not the same as comparing, for example, Jesus with Buddha. "Paganism" is actually a general term for some thousands of different religions from every region of the world. If you find even one element of Christianity in pagan religion #10,497, another element in pagan religion #8,966, and yet another in pagan religion #11,022, and so forth, you will eventually be able to say, "Look at all the similarities between Christianity and Paganism!" It is a completely misleading conclusion.