Spaced Sheathing Damage
If you're attic look something like
this, there's a good chance your home was built before the 1970s.
Spaced sheathing roofs were used for wood and shake shingles. Later to be replaced by solid plywood roofs.
Spaced sheathing can't provide the engineering requirements for modern-day construction and is no longer used.
If you notice, some of the damage to the sheathing is from pieces of the 1x materials that are split and sections are missing.This isn't common and could provide your roof with weak spots. While walking on a roof like this, be extremely careful.
Now for the big question, How can I fix something like this? I wouldn't worry about it until you were ready to re-roof the home. At that time, you can remove all of the 1 x 6 and install new plywood or if your local building code allows, install new plywood over the 1 x 6, after you have removed the roofing materials.
If you have spaces or gap's in between sheathing larger than 8 inches from damage, you could always fill this area with 1 x 6 or other building materials.
These areas should be filled and repaired for safety. You don't want someone falling through the roof, especially yourself.
If you choose not to repair this damage, make sure that you warn anyone who will be walking on your roof.