John Hammond, the owner of Jurassic Park, invited several experts in the field to help evaluate the operation of the the park. Where everybody's role was quite obvious, Dr. Malcolm's role wasn't so clear. He appeared to be a skeptic, and went about explaining chaos theory to effectively say, life will find a way. Still, John even expresses his abhorrence toward Dr. Malcolm during the early parts of the tour. So, that begs the question, what value does Dr. Malcolm provide?
In the novel (and forgive me here, it's been many years) several scientists were invited to get an early look. The paleontologists mostly so that they could critique anything that was wrong or anachronistic. Remember, they weren't sure that they'd "gotten the dinosaurs right", they did have to fill in some missing pieces so to speak. Malcolm though was a young and hip black mathematician (which is why Jeff Goldblum was the perfect casting choice) and he would have been included because he was famous and popular (at that time). That he was brilliant was just a bonus, and though interdisciplinary collaboration can sometimes lead to insights that would never otherwise be realized they probably weren't counting on that.
Furthermore, Ian Malcolm is a character that Crichton used extensively. He's a central character in Sphere, and though I can't remember which, in another novel.
In the real world, they'd probably invite Kim Kardashian to get a first peek at the park. You shouldn't expect that people always have rational reasons for the things that they do, especially executive management of secretive companies.
The novel describes the scientists who were asked to consult on the "new museum" that InGen were planning. Doctor Grant seems to consider them an unremarkable group of high-brow consultants for this sort of project.
“Yes,” Grant said. “He was planning a museum for children, and he wanted to feature baby dinosaurs. He said he was hiring a number of academic consultants, and named them. There were paleontologists like me, and a mathematician from Texas named Ian Malcolm, and a couple of ecologists. A systems analyst. Good group.”
Genarro, rather than Hammond is responsible for choosing the group to tour Jurassic Park. Malcolm is already known to be completely sceptical of the success of the project:
None of them had much to do with the island, and one of them-the mathematician, Ian Malcolm-was openly hostile to the project from the start. Insisted it would never work, could never work.”
Ian Malcolm was one of the most famous of the new generation of mathematicians who were openly interested in “how the real world works.” These scholars broke with the cloistered tradition of mathematics in several important ways. For one thing, they used computers constantly, a practice traditional mathematicians frowned on. For another, they worked almost exclusively with nonlinear equations, in the emerging field called chaos theory. For a third, they appeared to care that their mathematics described something that actually existed in the real world.
Malcolm was evidently selected because of his ability to distill real-world problems into mathematical outcomes. Hammond seems to enjoy sparring with him verbally and is very keen to prove him wrong by showing him a fully functional park.
With even their most ardent detractor (a 'rock-star' mathematician, no less) admitting that the park is going to be a success, what better advert could they have?