Next-Vote in Protocol

can someone explain the purpose if next votes when a block has already been certified by Certified-Votes

At a very high level, next-votes are only used when blocks have yet not been certified (this may not be completely true but this is gist of it).
This can happen for example when the block proposer is malicious and creates two different blocks and send one block to some participation nodes and the other block to the other participation nodes.

When the block proposer is honest, 80% of the participation nodes are honest, and the network is not partitioned, you only see soft-votes followed by cert-votes.