You can use "Card Guard" to protect monsters that can't be destroyed in battle, such as "Blackwing Armor Master".
You can place the Guard Counter on another "Card Guard" to have its ATK become 2200. Or swap Guard Counters between 2 Card Guards to have their ATK maintained at 1900 but with the added protective effect of the counter.
This card is useful in conjunction with "Burden of the Mighty". You can play "Burden of the Mighty", put "Card Guard's", "Guard Counter" on "Burden of the Mighty" to prevent it from being destroyed once. "Card Guard" will drop to 1600 attack, but even level 4 1900 attack monsters wouldn't be able to destroy "Card Guard" at that point.
You can Special Summon this card from your Graveyard with the effect of "The Dark Creator", and place all Guard Counters on "The Dark Creator" to create a hard to destroy monster.
Use the effect to protect "Wave-Motion Cannon", allowing it to last longer to build up it's burn power.
You can use the Guard Counter to protect already hard to destroy cards such as "Marshmallon". It can't be destroyed by battle, and the Guard Counter will protect it from one effect that would destroy it.