An idle session can be a blocking session. For example

Session 1:

insert into my_table (pk) values (1);

(The column "pk" is the primary key column)

This session is now idle, because the insert has completed.

Session 2:

insert into my_table (pk) values (1);

will block, because it must wait for session 1 to commit or rollback to see if it is allowed to add the same primary key, or raise an error about duplicates.