Pull and push concept is used for locomotive.
Various reasons may exist behind it. But the main reason is that When you pull some thing towards your direction more power is required with high friction.
But when you push some thing from behind ,friction is comparitively lower then pulling.
In case of locomotive Front panto means
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(High Friction) & Rear panto means Pushing (comparitively lower then pulling).
Correct me If Wrogn.

In principle there is no difference between both. Technically, both pantos are equally well suited for moving trains and there is no advantage one gives over the other in terms of power or effort.
The most commonly agreed upon reason is that if the pantograph gets entangled with the OHE and breaks off, the rear panto will not cause any damage to the rest of the roof-top equipment and on most occasions, the loco will be able to move ahead with it's front panto. If the front panto breaks off, however, there will very likely be a lot of damage to the rooftop equipment rendering the locomotive useless.
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A second, less commonly mentioned point is that the frequent arcing between the panto and the OHE can be distracting to the LP (The flashes of light one sees around the panto). Particularly in the night time, these bright flashes may cause some inconvenience to the LPs, so the rear panto is usually preferred.
Now in some cases (even in one of our FAQ questions on IRI), it is claimed that the front panto is used in winters to clear frost and dew from the OHE wire so that it does not fall on the rooftop equipment and cause a short circuit, but this does not hold up to scrutiny. Electric locomotives work without issue in heavy rainfall and even snow, so they are not really affected by pieces of frost or dew droplets falling on the roof.
However, in colder countries, occasionally both pantos are used in cold weather to clear up frost from the OHE wires. The front panto is isolated (does not supply power to the loco) and raised along with the rear panto. In this one the front panto clears up the frost on the wires and provides a better contact for the rear panto.