Why led bulb is hot to touch?

Because led generate much heat during working. The heat will dissipate through led aluminum PCB and aluminium heat sink. When the heat conduct to the housing, it feels hot to touch.

It is good for prolong the lifespan of led.

If the aluminium heat sink is not enough for heat dissipation, e.g., if 15 watt led using 10 watt heat sink, the temperature of housing will be very hot during working. And the led will have light decay sooner.

Best temperature for housing is 50-60 Celsius degree.

If too high and too low, will be not good for led.