The Torah tells us that there was a great famine around the world and people came from all over to buy food and grain in Egypt. Then it says that Yaakov saw there were provisions in Egypt and he said to his kids, "Why do you look conspicuous?" (the word Tisra'u seems to be under debate as to it's meaning, but I am using ArtScroll's translation, based on Rashi's pshat).
Rashi tells us that Yaakov was saying to his children that they should go down to Egypt now while they still had food as it is improper for a Jew to appear before the children of Esau and Yishmael as being needy.
The appearance of a Jew is very important. A Jew is obligated to always make a kiddush Hashem and not make a chillul Hashem. If the Egyptians and others would see the Jews suffering from hunger, it would be a great chillul Hashem, as that could be taken to suggest that the brachos did not come true, or that Hashem does not take care of the Jews.
Yaakov was therefore very aware of the impression his children would give over to the goyim. Better not to wait until the food runs out and then go to buy food, but buy the food in advance, so the goyim will not look down on the Jews.