Full grain leather is the top layer of the cow hide and far superior to the other layers that you may know as leather.
The marks / lines are not defects - it is just like the stretching / bruising / scarring you have on your own skin - it is what makes the products unique - as no two animals will be the same.
Other leathers are buffed to remove these marks - depending on the end use of the leather, it may be necessary - but all the leather ends up looking the same, erasing the life story of the animal the leather came from.