We need daily blood sugar levels to maintain control,yes,but unless you where constantly testing hour by hour you may be unaware of 'spikes' in your levels.This is where knowing your hba1c comes in handy because it shows the percentage over 90 days.You may think that your daily readings are good because they show say a 6 when you take them but your Hba1c may be 7% which would indicate that ,during your day you are actually getting higher levels at some points of the day.This helps you decide whether to do more one hour ,two hour tests for a while to iron out the 'spikes' in your levels.