I know nothing about the military, so I apologize if this is obvious. Why is this practical?
CRUSHER: I recommend we evacuate and seal off all non-operational areas, and group the families and crew on the odd-numbered decks.
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The ship is experiencing a rapid energy drain and is operating on reserve power. Since we know that a very sizeable chunk of the ship's energy output is channelled into keeping the crew alive (59.2%?), taking out half of the ship's decks would result in a dramatic lowering of their energy needs and usage for heating, gravity control, lighting, life-support, etc and hence increase the length of time that the shields can remain up.
GEORDI: With the engines idling, the energy loss has been limited. But our reserves will be depleted in less than three hours. We won't be able to hold our shields in place.
As to why they have them move to the odd-numbered decks (rather than having them all cluster in the centre of the ship, for example) one assumes this is because a) Now only half the ship's crew have to move at all b) The half that have to move only have to move up or down one deck, minimising turbolift use.
(h/t to ApproachingDarknessFish)
Using odd numbered decks is simple, convenient, and easy to communicate.
The key point here is to reduce the number of occupied decks. By specifying odd-numbered decks, people only have to move one deck up or down. Only the people on even-numbered decks have to move at all and it is easy for people to verify for themselves that they have moved to a safe deck. This simplicity should make the evacuation process easier and reduce the risk of anybody being on an unsafe deck by mistake. If you add the additional instruction that people on even-numbered decks move one deck up, then everybody is moving in the same direction.
It is a bit like the problem of separating a group of people into two equal sized sub-groups. If you just say get into pairs, they will take forever. If you line them up and give them each a number, it is much easier to divide them up as you wish.
One good reason would be to minimise risk if the situation escalates further.
Escape pods are evenly distributed across the ship.
Getting people onto even-numbered decks means that if they need to evacuate, then people can get to the escape pods quickly and efficiently, and with minimal overcrowding.
On the other hand, if everyone is moved to the upper decks and you need to evacuate, the escape pods on the lower decks would go unused and the ones on the upper deck would be overcrowded.