Because there has to be a set point at which to measure a dating system, else we'd have to start at the beginning of time, and be in the year 13,727,382,016 or something like that. A number that would be difficult to pin down for the ancients without use of radiocarbon dating or telescopes powerful enough to measure the cosmic background radiation or however we know how old the universe is. So we use a date that is significant to human history as a 0 point, but still need to be able to talk about things that happened before that point, so we go into negative numbers. Is that what you're asking? Or do you mean to ask why we use the approximate date of the birth of Christ? Or something like that?