I'd guess that because you are driving illegally , if you were to have an accident your insurance would not pay. That could be pretty expensive .
Not only that, but if you were pulled over by the police, you could be charged with both driving without a valid licence and also driving without valid insurance.Edited to add
: This is actually a standard thing - if you are stopped for drink-driving, for example, and are found to be over the limit, you will be charged with 3 offences:
1) Driving under the influence
2) Driving without a valid licence
3) Driving without valid insurance
Then, you have so many days to provide evidence of your valid licence and your valid insurance and they will drop the 2nd and 3rd charges (this happened to me when I was stopped in 2007).