system
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human
Why do they stop space launch countdowns?

I'm not sure that this is totally appropriate here, but I couldn't think where else to ask.

Sometimes, when reading about space launches(specifically NASA in this case) I see a statement like this:

"We're going to count down to T-2, then we're going to stop the countdown there for fifteen minutes while we work on something."

I don't get the stopping of the countdown.  Isn't the point of the countdown to tell you how much time you have before something happens?  Why stop the clock at T-2 for 15 minutes?  Why not just make the countdown T-17?  Wouldn't that be more useful?  A stopped T-2 could start in two minutes or in two days.
gpt
Everything must go as close to perfect as it can.

There is a T clock - T for test, where everything is checked against a list and the T clock has built in holds for double checks, confirmations and go/no-go decisions.

The L clock - L for launch, is the time remaining before the scheduled launch. Because of orbital mechanics issues, sometimes delays will exceed the launch window and there has to be a delay until the next day.

The US Space Shuttle had holds at T-27 hours, T-19 hours, T-11 hours, T-6 hours, T-3 hours, T-20 minutes, and T-9 minutes.

For Shuttle, the T-9 hold came up the most because during launch coverage the T-9 was usually during the live broadcasts. During the T-9 NASA would do the following:

* During the hold, final launch window determination

* Activate flight recorders

* Final "go/no-go" launch polls conducted by NASA Test Director, Mission Management Team and launch director

So why is there a T-2 hold - to do all the last minute things, the verifications, flight data recorders, get the radio telemetry going, fire up APUs, all sorts of things.

Here is a list of all the stuff they had to do past T-9 for Shuttle

* Once the countdown resumes, start automatic ground launch sequencer

* Retract orbiter access arm (T-7 minutes, 30 seconds)

* Start auxiliary power units (T-5 minutes, 0 seconds)

* Arm solid rocket booster range safety safe and arm devices (T-5 minutes, 0 seconds)

* Start orbiter aerosurface profile test, followed by main engine gimbal profile test (T-3 minutes, 55 seconds)

* Retract gaseous oxygen vent arm, or "beanie cap" (T-2 minutes, 55 seconds)

* Orbiter transfers from ground to internal power (T-50 seconds)

* Ground launch sequencer is go for auto sequence start (T-31 seconds)

* Activate launch pad sound suppression system (T-16 seconds)

* Activate main engine hydrogen burnoff system (T-10 seconds)

* Main engine start (T-6.6 seconds)