Hello Augustina,
The replies so far are accurate, but you may find there is a logistical advantage to purchasing in advance at a train station or SNCF boutique in Paris.
I just mean that there is no need to wait until day of. You can decide when you will stand on line for the ticket, potentially useful if your travel day is rushed.
If you happen to be near a train station you could buy your ticket in advance. Just make sure the agent knows you will not be using it that day, then validate the ticket at the station when you leave.
Of course for some buying the ticket at the station just before departure may be the most cinvenient way. just make sure to leave enough time to buy your ticket; wait times can vary.