After you've succesfully  withstands a zombie attack, you get a special bonus for selling all items in your store.

And as long as none of your buildings are destroyed, you will get the 3 hour Bonus.

So make sure your inventory is filled up to its maximum limit with having the variety of your best tier items.

Tip: make sure you know how to lock your best items (to keep them for the horde bonus sale) and how to offer other items if you don’t want to sell something requested by a customer.

Another great thing is that each building that survived after the defense receives a special buff: 

• Master buildings - acceleration of crafting corresponding items;
• Resource generators - acceleration of resource generation;
• Training camp - bonus to character experience;
• Canteen - reduction of recover time after Supply Runs;
• Warehouse - increase in the number of reagents obtained in Supply Runs; 

On the other hand if any of your buildings have been destroyed, you'll get a corresponding debuff.