In order for conception to occur, there has to be an egg (or ova) present for the sperm to fertilize, where the 'average' woman will ovulate around day 14 (from the first day of the last menstrual period).
Of course, not every woman is average. Because the human ova is viable for a short period (~24 hours), knowing the right 'window' of time is helpful if you wish to become pregnant. The calendar method can (and often is) used in conjunction with basal body temperature measurements and the examination of mucus, and is based upon statistical analysis studies. Some calendars will show a fertility window for the next 3 months, allowing for more 'planning' of bedroom fun. (Vacation, honeymoon, etc)
I'll be honest, it's much easier for me to plunk down my information on a calendar, hit "ENTER" and get a nifty calendar in front of me for the next 3 months, than to pull out my desk calendar, find the first day of my last menstrual period, then count forward to when my next should arrive, then backwards again 14 days, then back another 4 days, etc....and to do that for the next three months? Call me lazy, I don't mind.
Common sense (and logic) says that if you want to get pregnant, you need to have sex. However, work, school, kids, and life in general can sometimes distract you - and having a calendar can give that extra 'nudge' in the right direction. (wink wink, say no more).
Is using the calendar the best option? No, charting BBT and purchasing an ovulation test are much more precise; however they aren't free - while an online calendar is. There are also women who aren't thrilled about taking and charting their temperature daily.
There are also women who utilize it for avoiding pregnancy - given the short lifespan of the ova, they know which days to avoid unprotected sex - this was called the rhythm method Current natural family planning methods utilize a similar approach, which avoids intercourse during the most fertile phase.
ETA: An overview of high vs low tech contraception