Because if you're doing something with the engine you can't reach in through the window and turn the key you have to get in sit down press the clutch turn the key then get out again, a pain in the a**e.
Starting the engine without stepping on the clutch pedal is better for the crankshaft thrust bearings. However, the clutch is so light that it really doesn't make any difference in engine longevity. It's a good habit just in case the transmission was left in gear.
Thrust bearing wear is a problem with very strong aftermarket clutches. Basically, you're working against the clutch's pressure plate when you step on the clutch pedal and cars with very strong aftermarket clutches can wear the bearing down since the engine is getting started and you don't have oil pressure yet. This is more of a problem with racing applications and less on daily driver applications.
Yes but its a cheap trick to make sure they depress the clutch which removes the ability of the driver to start the car correctly as you should check it in neutral before hand. Automatics have had this feature since they were first designed you can't start the engine unless it in park or neutral.
I will add now what Chitty has mensioned. It annoys me when I sit in with drivers and they are waiting at traffic lights or some other road obstruction and they are in gear with the clutch depressed. Or they will sit in traffic on a hill riding the clutch rather that applying the handbrake thats even worse. As Chitty has said this is putting extra load/wear on the crankshaft thrust bearings. If you want longevity of the engine don't do it.
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