when often do we need a bar graph ??

For correct info

yes.

go , ,,,,,, seeee the boooooooooooook
 

FOR CORRECT INFO

when we do geometery

It depends upon you that you have to show it in statements or in a bar graph. It depends upon your idea that in which way you can show your information in neat 'n clean way, in simple words, in the best way. According to me, you should use statement way in long questions like how many people traveled during this,this,this and this year if there were large amount of people. And bar graph way in short questions as it is a very time taking process (when you have to simply draw the bar graph without using a bar graph paper) like drawing lines,making equal columns,etc.

BarGraph is needed when you want to represent the data in a Bar graphical manner. This method is simple for organizing data...

You can use it when there is not a more convenient method other than the bar graph.

ANSWER
Bar graphs are ideal for comparing two or more values, or values over time. Data is displayed either horizontally or vertically.

You may need it to see your status in different subjects,or a shopkeeper can need it to review his sales
It can be helpful to a teacher to compare the marks of her students,and many more uses are there