You may not have noticed, but 2020 completely changed the way we view Spinosaurus. Scientists have speculated for decades if it was a terrestrial animal or if the crocodile-like snout, with all its typical adaptations for fish eaters, meant that it was semi-aquatic. There was even speculation that its sail was used for herding swarms of fish into a "bait ball", because it looks exactly like the sail of some swordfish, who use it for the exact same thing. Nobody could tell for certain, though, because the only known Spinosaurus skeleton was incomplete, and also had been destroyed in an air raid in WWII. Paleontologists had nothing to go by except for the drawings of this lost specimen. It took until the 1990s until new Spinosaur fossils were found in Egypt, Morocco and Tunisia, but all of these were fragmentary.
Things took a change recently, though. A nearly complete tail and some other missing parts were found in the Kem Kem fossil beds in Morocco, and they revealed that Spinosaurus not only had shorter hind legs than expected, but also a keeled, paddle-like tail as we see nowadays in some sea serpent species. A robotic model has now shown that it enabled a way of swimming that was both fast and energy efficient. Given that so many other large carnivores like Carcharodontosaurus lived in the same area at the same time, it makes sense that they occupied different niches in their ecosystem of tidal flats and mangrove forests, and Northern Africa had an abundance of prey for a semi-aquatic giant dinosaur at that time, as you can see from the picture. All of these animals where found in the same formations as Spinosaurus fossils, from the giant bichirs and lungfish in the upper right to the chonky coelacanth Mawsonia (no. 5) and the sawfish Onchopristis (no. 9).
The sail is puzzling. I mean, crocodiles nowadays are largely ambush predators, a big sail poking out of the water when they're submerged just below the surface would be a real problem for them, giving away their hiding spot. I would expect either a different hunting method (like complete underwater hunting as seen in garials, or a herding of their prey as in sailfish or thresher sharks) or, if it was an ambush predator, a different posture than a crocodile, like being submerged at a steeper angle so the sail is underwater while the nostrils are poking out, and then propelling itself upwards with the tail. There's always been theories about the sail helping with thermoregulation, as with Dimetrodon and other synapsids from the Permian age. There have been theories in the past that the spines supported a fatty hump for energy storage, as is seen in camels and bisons. But recently, it looks as if it didn't support a hump and would rather have helped to stabilize the animal while swimming. I mean, that exact sail shape does work pretty well for sailfish, but they also have a completely different body plan and are ridiculously fast swimmers adapted for a life in the open ocean. I think there's plans to build a robotic model of the completely animal, not just the tail, once researchers have figured out more about the anatomy of its limbs, if it could have had webbed toes, how far it could stretch hands and feet etc., to find out more about how it moved around in the water. That could clear a lot of things up. Or just open up more questions.
Of course, having a large structure on your back that makes you look even more enormous than you already are could be useful when you want to intimidate other large carnivores. I could see a Spinosaurus chasing other theropods away from their kills, like lions do with hyenas and other smaller predators in their environment. There were large sauropods like Paralititan and large sauropod hunting specialists like Carcharodontosaurus in the same environment, and these other predators were a good deal smaller than Spinosaurus. Chasing them away to get some free sauropod meat sounds like a nice addition to a Spinosaurus diet.