Great question! It's tempting to speculate what certain features of plants and animals "are for," that is, what is their function and why did they arise? Such questions can be notoriously difficult to answer, even for modern forms that we can easily study. For example, why do elephants have big ears? I doubt we have an adequate explanation for this. Do big ears help elephants hear, attract mates, scare off predators (or do they serve no particular function at all? ) --it's difficult to test the various alternative hypotheses. Questions of this kind are even trickier to answer for extinct forms, such as spinosaurus, since living representatives aren't available for study. Some people think the sail in spinosaurus was involved in thermoregulation, or for display, but we'll almost certainly never know for sure.

Nobody really knows. It's possible that it was for heat regulation like the ears of an elephant, or as a display surface like the tail of a peacock, but it's hard to know for sure.