When does my engine need to be set at top dead center?

Ok, so i am teaching myself to do a head gasket replacement, i have a car manual and i can find a multitude of answers on how to set the pistons on my ford explorer to top dead center however nowhere does it explain WHEN and WHY to put it at top dead center
when replacing the head gasket on my car i know do not have to remove the timing chain/belt. I have already removed the ignition coil pack (instead of distributor) and the intake manifold.
i was wondering whether it is necessary to put the piston #1 at top dead center before i remove the valve covers, rocker arms andamp; pushrods, and cylinder heads, and why it is necessary
what relevance does this procedure have? Is it safe to turn the crankshaft when the ignition coil pack and intake manifold are already taken out?
thanks

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