The decorations are there to help your dinosaurs gain more coins, though not all decorations have a high give-away-percentage. They can be used not only in the main Jurassic Park, but also in the Aquatic and Glacial Parks. The coverage (range) of decorations are mostly small and may only cover one dinosaur at a time, but there are some that have a huge range and give a high percentage (though those cost quite a bit of coinage).
Using them wisely can help you get up to eight dinosaurs to benefit from their percentage gain!