If you're chart shows high temperatures for three days longer than your longest observed luteal phase, or if you have 18 high temperatures in a row, there's a good chance you are pregnant. Of course, you may already know by that point, since pregnancy tests are generally accurate by the 12th day after ovulation!
Charting has the added benefit that it's easy to calculate your due date. Pregnancy is generally measured from day 1 of the last menstrual period, because most women don't know when they ovulated. The average length of time from last period to babies arrival is around 280 days (40 weeks), and if the baby hasn't arrived at 294 days (42 weeks), doctors start to worry about the health of the baby and strongly recommend induction or c-sections for delivery. This means that having an accurate idea of when you got pregnant can be very important to getting appropriate health care for you and your little one. Additionally, knowing when you ovulated is the best way to date a pregnancy - more accurate than ultrasounds even, because it works even when the pregnancy has complications that may impact healthy growth.
To calculate, take the date you ovulated, subtract 14 days, since the assumption is that ovulation always happens on day 14 of the cycle (LOL). Then add 280 days. This is your estimated due date.