Air India — Booked seats on flight changed without notice

I booked a return ticket with Air India from Toronto to Delhi and booked a bassinet seat for me and my infant baby with a nearby seat for my wife. The airlines changed my seat without any notifications to a seat which was completely away from my wife's seat and for the return journey removed me from my seat. The reason provided by airlines was that the aircraft was changed. But if the aircraft was changed, they should have kept the seats together like they were booked. What is the point of booking the seats in advance then?