I own a handful of Razorbacks, but I honestly can't recall a single time I've used one in the last five years.
As a transport vehicle, the Rhino is far and away superior. It exists for the sole purpose of advancing your Space Marines without worrying about small arms fire. The Razorback can't carry a full squad, which makes it worthless as a transport, in my opinion-- a Combat Squad isn't a serious threat, but ten Space Marines are a problem you have to devote heavier resources to counter.
Sure, the Razorback comes with a good weapon, and it's functional as a heavy weapons platform. If a squad you bring has no need for a transport vehicle, such as a Devastator Squad, then a Razorback has a place in the army as a suppliment to their firepower. However, its points aren't meaningfully less than a Predator in the grand scheme of things, and a Predator is exponentially better. Unless points are really tight, you're better off just bringing a Predator in lieu of the Razorback.
Once you've unloaded a Rhino, it actually can be a pest for your opponent-- since it really is disposable at that point, you can put it in hazardous situations you wouldn't dare put a Razorback in, since you'd want the Razorback to keep firing. The Rhino can be used to block line of sight from enemy heavy weapons teams, pepper units with its storm bolter, block fall back routes from close combat for the enemy (so if you can break them somehow, they'll be trapped and destroyed), and funnel enemy units into chokepoints by blocking off clear avenues of assault.