I've solved many problems calculating the electric field, $E$, of a given charge distribution but I'm just musing a bit as to why this is useful. The definition of $E$ involves the force, $F$, on a test charge, $q$, and the ratio $F/q$ in the limit as $q-> 0$. So if we have an $E$ field and place a non-zero charge in it, unless the source distribution is glued in place and/or the charge is very far away the charge will cause redistribution of the source charges and the resultant force on the introduced charge will not be $qE$ using the $E$ originally calculated.
So why is the concept of the electric field so useful? Of course, we could dispense with the $E$ field and directly compute the Coulomb force on a non-zero charge due to a given configuration of other charges, but how realistic are all these problems if the effect of the "test" charge on the source configuration is neglected?