More neurons are needed to control their larger muscle mass and to collect data from their extra sensors. It seems that one side effect of having a bigger brain is to be more intelligent. Perhaps this is because the number of possible interconnections between neurons rises as the square of the number of neurons. So it seems that evolution has led to increasing intelligence simply because it has led to increasing body size. Vertebrates can be bigger than insects and nematodes because, among other adaptations, they have internal skeletons and oxygen-carrying blood.
Greater intelligence is a by-product.
Yet, in spite of these considerations, it is clear that humans have a bigger brain than would be expected for a mammal of our size. The traditional explanation is that our greatly increased intelligence gave humans a survival advantage, but there's little evidence for that. If extra intelligence helped our ancestors survive, then we would expect evidence in the form of increasingly sophisticated tools. That doesn't seem to have happened. Homo habilis 's tools stayed exactly the same for a million years, then Homo erectus 's tools stayed the same for another million years.