Even though your tyres still have a tread, it might not be deep enough for safe grip or braking in the wet or just even in general. Depending on the baldness of the tyre. This is where treadwear indicators come in handy.
In the grooves of every tyre you’ll find little bumps called treadwear indicators, which tell you when the tread is no longer deep enough to provide sufficient grip and braking distance, particularly in the wet. If your tyres are bald you will just slide & this is dangerous.
Watch this short video to find out more about treadwear indicators and other signs your tyres are becoming unsafe.