Knowing when you are most fertile is the best way to know when to get pregnant within your cycle. Every women is different and it’s simply not always a case that you ovulate on Day 14 (remember this is just an average), so by charting your daily basal body temperature you will start to get a good indication of your usual ovulation day.
Charting should be used in conjunction with the other methods of determining your ovulation day because while useful, it will only confirm that you have ovulated, not tell you when you are about to. However after around two months of charting you should have a good idea when the best time to get pregnant in your cycle will be.
It’s also useful for detecting luteal phase defects (when the time between ovulation and menstruation is less than 10 days).