In the movie Ice Age, Scrat has two main goals.
One is finding acorn and then burying it. Why does he want to bury it in the ground?
Because Squirrels do that. It's their way to hide food for the cold. If you google about it, you will find many articles about it and one such article from ssec.si.edu say :
If you live in a rural area, or near a park, you’ve probably observed this unique squirrel behavior. When the weather catches a chill, these bushy-tailed creatures begin what looks like preparation for a wide-scale scavenger hunt. In great numbers, these squirrels begin to bury nuts! Squirrels hide nuts this way as preparation for cold weather when otherwise food will be scarce.
And from brainstuffshow.com:
Not all squirrels bury nuts. Gray squirrels do bury things like acorns and walnuts, but red squirrels do not. Red squirrels horde their nuts in piles. As described in this article, these two different behaviors can have a big effect on a forest. Gray squirrels do not recover all of the nuts that they bury, and these nuts go on to become trees.
And Scrat is a squirrel living in Ice Age, so this behavior looks parallel to behavior of grey squirrel.