Node's expired SSL Certificate

[jindalcdac]

Good afternoon, i have some question regarding the expired ssl certificate of the node :

1. Can we join that node to cluster ?
2. Can we migrate that node vm to another node ?
3. Can cluster remain healthy ?
4. Can we create VM in a node (expired ssl certificate) ?

If answers to the above questions is "yes", then i need to know why are we using certificate then ? If its expired, node communication is happening in encrypted format, I can login to that node easily and encrypted traffic is going on.
Please tell me i am waiting for your response.