We know that our bike only makes contact with the road via the tyres. The tyre pressure is an important safety factor, but the tyre profile also plays a crucial role in the behaviour of your motorcycle.
In most European countries the minimum legal tread depth for motorcycle tyres on public roads is 1.6mm. Remember seeing that figure at some time during your motorcycle driving lessons? Of course, as a responsible driver you don't have to wait until you only just still meet the legal requirements.
On the contrary, you should know that as soon as the tread depth of a motorcycle tyre drops below 3 mm, it starts to become less efficient in draining away water. This reduces the wet-weather grip of the tyre. Don't forget: the tyres - just two fistfuls of it at any one time - are your only contact with the road. A worn-out tyre with almost no tread depth left significantly increases the risk of aquaplaning. It also increases the braking distance in wet conditions.
Regular measuring of the tyre profile is therefore strongly recommended. Measure it, don't just estimate it. It is hard to see the difference between 'still okay' and 'time for a new tyre'.