On my opinion, the following citation from Poincare answers this question:
“At the time, when Maxwell initiated his studies,
the laws of electrodynamics adopted before him explained
all known phenomena. He started his work
not because some new experiment limited the importance
of these laws. But, considering them from a
new standpoint, Maxwell noticed that the equations
became more symmetric, when a certain term was introduced
into them, although, on the other hand, this
term was too small to give rise to phenomena, that
could be estimated by the previous methods.
A priori ideas of Maxwell are known to have waited
for their experimental confirmation for twenty years;
if you prefer another expression, — Maxwell anticipated
the experiment by twenty years". (H. Poincare, The value of Science).
Maxwell's contribution was introduction of the so-called "displacement current".
Then his equations predicted the existence of electromagnetic waves, the thing experimentally confirmed by Hertz.