Warm tyres have more grip so heating them before they go on the car means that they are in the window immediately.
In sports categories where tyre warmers are not used, the driver needs to warm the tyres by spinning them or weaving on the track to create heat through friction.
Tyre warmers, or tyre blankets, are essentially electric blankets that heats the tyres to 100C (212F).
They have been a major topic of discussion and are likely to be phased out to reduce high energy consumption, and costs,  placing the onus of bringing the tyres into the operating window back onto the driver.
[caption id="attachment_6097" align="aligncenter" width="1200"] Tyre Blankets - f1chronicle.com[/caption]