Hello Narad
The neighbor activate command is used to activate a neighbor. By default, the exchange of addresses with BGP neighbors is enabled for IPv4 address family by default. It is disabled by default for all other address families, such as IPv6. So in this particular case, the command is necessary. Since IPv4 is being used for a neighbor peering to exchange IPv6 addresses, and since this is all configured within the IPv6 address family mode, this command must be applied. More information about it can be found here:
Now It is obviously cleaner and more consistent to create an IPv6 adjacency with IPv6 prefixes. However, this is not always possible. What if you have a connection between BGP peers that is using IPv4? Such a configuration is not applied as best practice, but out of necessity.
I hope this has been helpful!
Laz