What are sessions and why store them (express-session)?

Question

Node.js

In short, what are sessions, why store them on the server?
What are they needed for? Practical application examples?
If they are only needed to identify an authorized user by the server, are they needed if JWT is used ? Roughly speaking, do I even need to express-sessionstore sessions if my authorization and access to private routes is based on JWT?
Or is their application wider? Explain clearly plz.