Here are a few reasons, which might be more or less compelling for you, depending on your own preferences:
Do not simply discount it for being "syntactic sugar". While you may say that something is just syntactic sugar, it is after all the sugar that sweetens your life - as a programmer just as well as a coffee or tea drinker.
Singletons - every Scala
object is inherently a singleton. Considering that in the Java world people are implementing singletons in all sorts of different ways and more often than not end up making some mistake in their implementation, you cannot make an error as simple like that in Scala. Writing
object instead of
class makes it a singleton and you're done.
Access to static methods: The static methods in Java can be accessed from objects. For example, suppose you have a class
C with a static method
f and an object
c of type
C. Then you should call
C.f, but Java allows you (albeit with a warning) to use
c.f, which when you come from the Scala background doesn't really make any sense, because objects do not have a method
Clear separation: In Java you can mix static and non-static attributes and methods in a class. If you work disciplined, this doesn't become a problem, however, if you (or someone else for that matter) do not, then you end up with static and non-static parts interleaved and it is hard to tell at a quick glance what's static and what's not. In Scala, everything that's located inside the companion object is cleary not part of the corresponding class's runtime objects, but is available from a static context. Vice versa, if it is written inside a class, it is available to instances of that class, but not from a static context. This becomes especially burdensome in Java, once you start adding static and non-static initializer blocks to your class. This can end up to be very hard to comprehend in terms of dynamic execution order. It's a lot clearer in Scala, where you initialize the companion object from top to bottom and then do the same for the class in case of a runtime object being created.
Less code: You don't need to add the word static to each and every attribute or method in an
object, thus keeping the code more concise (indeed, not a prominent advantage really).
Disadvantages are much harder to find. One might argue, that the static and non-static parts should belong together, but are separated by the Scala concept of companion objects. For example, it may appear strange to have a class diagram, but then end up having to create two things in the code and dissect which attribute goes where.