When should we use Custom Session Mode ?

we can use custom session mode in following of the cases,

• We want to store session data rather than SQL Server.
• When we have to use some Existing table to store session data.
• When we need to create our own session ID.

What configuration do we need for it?  

We need to configure our web.config like below,

If you want to Explore some thing more please Check Further study section

Advantages and Disadvantages 

Advantages :

• We can use some existing Table for the store session data, It is useful when we have to use some old database rather than SQL Server.
• It's not depending on IIS , So Restarting web server does not make any effects on session data.
• We can crate our own algorithm for generating Session ID.

Disadvantages :

• Processing of Data is very slow.
• Creating a custom state provider is a low-level task that needs to be handled carefully to ensure security.

Its always recommended to use any third party provider rather than creating your own. 

Ref and More Information : Custom Mode