Why does the time depth of prehistory matter to us today?
Since you have placed this in the History section, I assume you are asking about this from a historical point of view. If so, the time depth of prehistory matters to us today because it tells us that there is a great deal that we do not know about human history.
Historians like to think that studying history can tell us a great deal about how human societies are likely to behave in given circumstances. Studying history tells us how humans tend to form their societies and what their societies tend to be like. But the depth of prehistory makes it so that conclusions we draw about this may not be as accurate as we hope. Human prehistory is so much longer than recorded human history. Therefore, there is a huge amount of time that we as historians cannot understand properly. This limits our ability to use our historical knowledge to make conclusions about how human societies behave.