It's to keep the hormone levels in your body constant. If you take them at random times you might end up with too much hormone at one point and not enough at another. Some women find they experience 'spotting' if they take their pills at different times. I find it helps my mood if I ensure I take my pill at the same time every day, but it's also important as I'm on a triphase pill so the amount of hormone increases as I move through the pack.

So they are working more effectively!

1) "Research and use by many women has shown that birth control pills are as much as 99.9% effective in preventing pregnancy when used perfectly. Perfect use refers to taking the pill at the exact same time every day (so that your hormone levels stay consistent) and not forgetting any pills. However, with typical use, the pill may be as low as 92% effective. Typical use means that you do not take the pill at the exact same time each day and that you might sometimes forget to take a pill." "It is important to take a pill at the same time everyday particularly if you are using the progesterone-only pill. If you delay the time by 3 hours or more, you might have to use another effective birth control method in addition to your pill. Linking your pill taking to a daily activity, like eating breakfast or brushing your teeth, will help you make it a part of your daily routine. Other women find it helpful to set a watch alarm so that they never forget to take their pill. If you do forget to take a pill, take the missed pill as soon as you remember and continue with your regular schedule. If you miss two or more pills, consult the instructions that come with your birth control pills." Source and further information: http://www.womens-health.co.uk/birth_control_pill.html 2) Here an example (usage can depend on the specific pill): "Forgotten Tablets Provided she is less than 12 hours late in taking her tablet the patient should take it as soon as she remembers. Further tablets should be taken at the usual time. Mercilon will still give contraceptive protection during this cycle. If she is more than 12 hours late in taking one or more tablets then she will not be protected for the next 7 days. If one or more tablets are forgotten for more than 12 hours, contraceptive protection will be reduced. The patient should take the last forgotten tablet, even if this means taking two tablets in one day, and then continue to take tablets at the normal time. Additional contraceptive precautions should be taken for the next 7 days, and the patient should follow the '7-day rule'." Source and further information: http://emc.medicines.org.uk/document.aspx?documentId=5384#POSOLOGY