How are the bridge bonds in $\ce{Al2Cl6}$ different from the terminal bonds?
We could have just mentioned the bridge bond formed by $\ce{Cl}$ as a coordinate bond. Why introduce this extra concept?
I can understand why 3 center-2 electron bond would have been introduced but not this one.
My guess is that during the formation of the molecule vacant orbitals take part in hybridisation which is different from usual but after formation of the bond, there is no difference.