when u want to hide the column based on some condition then 
we will go to the render variable option.

Render variable is something like hiding(rendering) 
anything using a variable(Condition)

As far as i know

Render variales are generally created to conditionally 
render a particular page or chart or list or block etc...

Though the purpose of the render variable is to hide the 
particular item..we can't say that it is use to hide a 
particular item...

Consider a scenario..
You have created two list reports for two countries

List1 is for both country1 and country2
List2 is for country 2 alone

List1 is created from the query1 
List2 is created from the query2

Now u create a conditional string variable called 'Country' 
which uses country name as a parameter 

Now u associate Variable 'Country' as render variable for 
List1 and List2

Now whenever u run the report..when u select country1 then 
automatically the two queries will be rendered to the two 
list and the two lists will be displayed

But when u select Country2 as parameter then only Query2 
will be rendered and the list2 will be displayed in cognos 
viewer window...

Note...

When u use Conditional variable u r just hiding the output 
from the user..but in the backend the two  queries will be 
processed by cognos....hence ur precious time will be 
wasted..

But when u r using the render variable only the Queries 
which satisfies the parameter will be rendered and hence 
the other queries will not be processed in backend by cognos

I hope this helps you