Just because your tread depth isn't worn out doesn't mean your tires don't need to be replaced...
Here's a case in point: Ninja 250R with 8,000 miles. Tread depth is fine, but look at the profile of the old tire on the left versus the new one on the right.
Additionally, check for dry rot on tires that have sat thorugh a winter without being used. Again, the tread might appear to be fine but that doesn't mean they shouldn't be replaced.