Suppose you believe, as many people do, that climate change due to anthropogenic CO2 is a serious problem. There are two different ways you might try to deal with it. One is by trying to keep it from happening, or at least to slow it. The other is by adapting to it. There are at least two respects in which the latter approach is superior to the former.
The first is that it avoids the public good problem. If the U.S. switches to more expensive sources of power in order to hold down CO2 output, any benefit from reduced warming is shared with the rest of the world. It is unlikely to happen unless either the benefit is so much larger than the cost that it is worth doing for the U.S. share alone or many countries manage to coordinate their policies, despite the obvious temptation for each to free ride on the efforts of the others. Neither is impossible, both are difficult.
Adaptation does not face that problem. If Bangladesh deals with sea level rise by diking its coast, the benefit goes to Bangladesh, not to the U.S. or China. If a farmer deals with an increase in temperature by shifting to a crop better suited to the new conditions, he gets the benefit.
The second advantage of adaptation is that it affects only the negative consequences of climate change. While the public discussion often obscures the fact, there are positive consequences as well—indeed, it is not clear that the net effect is negative, especially at low levels of warming. Milder winters are, on the whole, a good thing. So are longer growing seasons. So is an expansion of the habitable area of the northern hemisphere, due to temperature contours shifting north. A reduction in warming eliminates the good consequences as well as the bad. Adaptation can target only the bad consequences.
Neither of these proves that adaptation is superior—that depends on the costs of adaptation, the costs imposed by warming, the benefits imposed by warming, the costs of reducing warming. But both are arguments in favor of adaptation.