The purpose is to provide freeview digital tv to those who can't get it through the aerial, nor from Sky or Virgin.
If you get freeview already, or can get it through the aerial with a set top box or digital tv, you dont need it.
It's explained in the article you linked to:
Emma Scott from Freesat told BBC Radio 5Live: "It's got 98% coverage across the UK and it's only homes that really don't have a clear line of sight to the satellite that can't get it, or certain listed buildings.
"But unlike Freeview you can receive Freesat across the country, which means that in those areas that at the moment can't get access to free digital television this is the first time that you can get free digital TV."