What is the main purpose of using colspan?

Answer 1

suppose u r having a table for person and and there are several attributes one of them is phone no.
so if any person have multiple phone numbers then we have to span whole columns

Answer 2

Colspan tells renderer that table cell should take space of some number of cells.
It's usable, when you have multi row table and some rows has less cells than others.

Answer 3

It is used to merge column.