Starting a voice chat at a set time helps members know in advance when a voice chat will start.
Scheduled voice chat link helps to inform the voice chat with the link on other social networks before starting
And The user enters the channel / group by clicking on this link and if the voice chat has not started, they will see the "Set reminder" option
And by clicking on this button, a message will be set reminder with the start time of the voice chat in "Saved messages" that the text of the message includes the voice chat link.
At the specified time, you will receive a reminder message in the saved messages
As of April 2021 admins of groups and channels can schedule a Voice Chat for a particular date and time instead of creating one right away. This gives community members time to let their friends know and get the popcorn ready.
You can find more information about that update in the blog post: https://telegram.org/blog/payments-2-0-scheduled-voice-chats