Cracks in Leather
The only way to un-crack leather is to replace it. One approach that leather repair specialists use is to clean the leather and then re-color it. The result is that the leather will have a uniform appearance. The cracks will still be there but will be less apparent.
You can also have a leather repair professional replace the cracked areas and match the new leather to the original color. This works best when you are working with a panel of leather, for example on a car seat or the surface of a furniture cushion.
Dog and Cat Scratches
If you own a dog or cat, do not allow them on your leather furniture PERIOD!
For dogs, most damage occurs while climbing on and jumping off. If you are a cat owner, leather fillers or tanners resin will cure the symptoms but not the problem.
You might consider buying your cat a scratching post and put it near the area where the cat is scratching before using the filler.
Spray water on the scratch and cover it with a small square of 1000 grit wet sand paper from the hardware store. Sand very lightly until the surface is smooth and you cannot feel the tear from the scratch anymore....