Glycolysis yields 2 net ATPs. However, gluconeogenesis (lactate->pyruvate->glucose) costs 6 ATP, with a total net loss of 4 ATP.
Does the body not care about the net loss because of the potential energy from glucose to yield up to 38 ATP?
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$\begingroup$ Welcome to SE Biology. As a new contributor you should do some research before posting here, as outlined in the Help on Asking Questions. In this case you might like to ask yourself (1) Why do cells make ATP if not to use it? (2) What is the purpose of gluconeogenesis by the liver — what is the product, for what tissues is it destined, and what would happen if it did not occur? Even if you cannot answer the question for yourself completely it would allow you to construct a more focussed question. $\endgroup$– DavidCommented Nov 16, 2018 at 15:49
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The brain and also the erythrocytes need glucose to survive. Gluconeogenesis is one of the possibilities of the body to keep the glucose levels stable and to avoid hypoglycemia. The precursors needed come from amino acid and lipid metabolism, and is turned on, when the body has no energy sources available which can release glucose, like carbohydrates in the food or the degradation of glycogen.
As you said, it comes at a price, making glucose itself costs a lot of energy. This process is available to maintain the level of glucose, which shows the importance of it.
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$\begingroup$ One point worth mentioning. Lactate produced by anaerobic utilization of glucose in erythrocytes can be recycled so there is little or no net requirement for glucose. The need of the brain is for net glucose as this is completely oxidized and no lactate is excreted from the tissue. $\endgroup$– DavidCommented Nov 16, 2018 at 15:56
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$\begingroup$ On another note- you might wish to reference this answer, or I'll banner it ;-) $\endgroup$– AliceD ♦Commented Nov 22, 2018 at 7:58