A time delay fuse is used in electrical appliances to allow an overloaded fuse to continue functioning for a short period of time before burning out completely. They are used to prevent fires in appliances that require a large amount of electricity on start up, and are designed as a safety mechanism to prevent the fuse from burning for too long.
As a time delay fuse is generally put into an appliance to help keep the users of the appliance safer, they are not interchangeable with other fuses to prevent the wrong type of fuse from being used. An example of an everyday application that uses a time delay fuse is a fluorescent lamp. The lamp requires a large amount of energy to turn on, but it does not require constant energy while in use. So while lighting the lamp may initially may overload the fuse, as it is a time delay fuse, it won't blow, which gives the energy surge a chance to level back out without posing a fire risk. Appliances that have coils or starter motors often use time delay fuses to help get them started and running without using constant power from the fuse.