New tires owe their shiny appearance to the products used for removing them from the mould - a lubricant called "mould release wax". This lubricant makes a new tire extremely slippery. This is why it is vital to gently run-in the tire for 30 to 50 Km - enough time for normal use to eliminate the mould release wax from the tire's surface, thus enabling it to achieve its normal grip.
During the first few dozen kilometres it is advisable to ride gently, and to ride as if you were on a slippery surface: do not accelerate, brake or change direction suddenly and take corners gradually.
The amount of a tire's wear is measured using wear indicators which are present in the tread. When the wear indicator is reached it is time to change the tire because this means that the tire has reached the legal minimum height of rubber. It is even advisable not to wait so long and to change the tire beforehand. Firstly, because with wear, the grooves in the tread lose their depth and thus lose their ability to evacuate water. In the rain, risks of aquaplaning are increased and there is less grip than with a new tire. Once the indicator has been reached, the tire loses its ability to evacuate water.
Next, independently of the depth of the tread, the tire loses its qualities with temperature cycles. Every time you ride your bike, the tire is subject to a temperature cycle: it warms up and then cools down. The constant repetition of these cycles end up changing the tire's qualities. In particular, it loses grip.
Lastly, even if it is not used, a tire gets old and hardens quite simply due to the passing of time. The tire's material and structure end up losing their qualities. This aging sometimes shows visually by cracking. This is why, if you buy a second hand bike which hasn't been used for a year or more, it is vital to change the tires even if they don't appear very worn.