Without a Will the fate of your assets will be determined by law and therefore people outside your family will not receive anything and more deserving members of your family may not receive what you would have wished for them. Secondly, non deserving members may take an equal share to those who have been the most generous and kindest to you.
If you have children under 18 years of age you can state in your Will how you want them cared for both personally and financially should you die before they reach the age of eighteen.