I entered my estimated tax payments for 2018. Why didn't this lower my tax due?

When I entered my estimated taxes paid for 2018, it didn't change my taxes due.  Why not?  What is the point of paying estimated taxes if it doesn't lower the amount you pay in April?  Also, the IRS never sent me a form.  I totaled up the amounts from my checkbook records.  The checks I sent them were all cashed.