Train ticket purchase in advance necessary in France?

For the trip to Le Mans, just to make sure that I understand the question: you have a 4:30 PM entry into the Eiffel Tower ticket and plans for a 6:30 PM train from Gare Montparnasse, correct? It's always doable to buy tickets at the train station but in your case, I would buy ahead of time not so much in case the prices are lower but to give you less to do between leaving the Eiffel Tower and getting on the train. I'd plan on taking the 7:30 train because you have too much scheduled in too little time with the 6:30 train. If you buy your tickets in advance they can be changed to the later train prior to the departure of the earlier train.

Thank you, JHK. Yes, we are planning to take train out of Gare Montparnasse for 6:30 departure after 4:30 entrance to Eiffel tower ( we are only going to 2nd floor) So I guess it is better to buy train ticket ahead of time for 6:30 and if we don' t make it then we can change when we get there ( after 6:30 I assume), change to 7:30 departure?

And would it be same for Bayeux - Paris? buy it and if we miss the train then change to other?

It is nice that they would let you change even if you miss the train.

I edited my reply. I meant the earlier train. My apologies. Will you have to go get luggage after the Tower? They don’t have luggage storage there. Are you planning on using mass transit? No matter what, I just don’t see going to the ET for a 4:30 ticket, going up to the second floor, leaving the ET, getting your luggage, and getting on a 6:30 PM train at Gare Montparnasse, but I wish you the best of luck.

In November 2021 it still took us 45 minutes of standing in line, with our reserved time entry, before we got to the elevator to head to the second floor of the Eiffel Tower. Even though it was misting & chilly, the crowds were out for the timed entry lines & walking up the stairs. Maybe the later 7:30 pm train, just to be safe, so that you can at least enjoy your time and views from the Tower.

Enjoy your trip.

If you buy your tickets in advance they can be changed to the later train prior to the departure of the earlier train.

So if you miss the train it is too late to change them at the station.

Thank you everyone for the response.
We don't have to worry about luggage. So it will take us to move diligently to the station. So then what if we buy ticket for 7:30 train but get there early to catch 6:30, then will it be possible to change earlier train? And how fast or difficult to change the ticket?
I would assume that you need to talk to someone at the ticket counter?

The French train system app lets you change tickets on it, assuming you are buying them that way. SNCF is the website and the app is easy to find.

It took us 70 minutes in April with timed tickets to get to the elevator at the Tower -- you do not have time to go up the tower and then make it to Montparnasse for a 6:30 train. If you manage it, it will be in a crazy rush. I'd buy the tickets last minute or get a later train.

Honestly there's almost no way you'll make the 6.30 and if you do, you'll spend your Eiffel Tower visit looking at your watch. The train ride to Le Mans is 1 hour, so does the 7.30 train make that much of a difference?
And yes, buy in advance both for Le Mans and for Bayeux.