Let's say you have a table called "user" with 1.000.000 records.
In this table you have a column named "banned". If the cell has a value of "true" this means that the user has been banned and can no longer access their account.
However out of the 1.000.000 users only 10.000 have ever been banned, which means that there are 990.000 records for which the storage that the column "banned" has been taking up has been somewhat wasted.
Wouldn't it make more sense to have a seperate table that stores the id's of the 10.000 users that have been banned, in order to save space?