CHEMISTRY COMMUNITY

I understand what a substitution reaction is (where one Lewis base takes the place of another) but how and why would this occur if there's already a Lewis base connected?

I am unsure of why the substitution reaction does occur; however, the bond between the acid and base breaks apart and the acid or base takes the place of the other by forming a coordination bond between the reactive centre and the new molecule. When I do my research, I have read that this process is very important within polymer analogous reactions to prepare functionalized polymers.