Each month, the follicular phase, or first part of your cycle, can fluctuate based on trivial things like travel, a cold, stress in your workplace, and many other day-to-day nuances. Even someone with a regular cycle can have a month with a longer or shorter follicular phase, leading to a surprise late period or an early period catching them off guard. The luteal phase stays consistent, only varying by one day max in either direction any given month, so it is the follicular phase that can get thrown off by small variables each cycle. The final days of the follicular phase make up your fertile window.
Let's say you usually have a 30 day cycle. This month, you are on cycle day 32 (CD32) and still no period. Your pregnancy tests are negative even though you are 2 days late, and that never happens. You do not chart, so you do not know how long your luteal phase usually lasts. Let's take a magnifying glass to what hypothetically happened during this cycle if you charted: In previous cycles, you ovulated on CD18, followed by a 12 day luteal phase, giving you a 30 day cycle. This month, you got the flu 2 weeks into your cycle. You did not think much of that in relation to ovulation, but when CD17 rolled around this month, your LH strips remained negative. You did not get a positive until CD20. Your BBT temps did not rise until CD22, and they have been consistently elevated ever since. Based on this information, you most likely ovulated on CD21, and you are currently 10 days past ovulation (10DPO). Since your luteal phase is 12 days, you will most likely start your period in 2 days on CD34.
This is exactly why charting gives you information that no app or calendar can. Instead of sitting in confusion wondering why you are late, you would know precisely where you are in your cycle and what to expect next.