I've used Spelljammer (heavily modified) in my homebrew since the 1990s. Here are a few ways it fits into my setting as a beneficial addition:
1.) Evolution of sailing: If you have a sea based campaign, Spelljammer gives you a way to evolve it to keep it interesting over the life of the campaign. As you reach higher levels, the change from 2D to 3D 'naval' combats opens up new considerations and options. In other words, it is not al alternative to sea based campaign - it is the second half of it.
2.) It fits better with moderate high level play: When you hit 9th to 11th level, the concerns of the PCs often start to tend more and more towards extraplanar activity. Your foes might be extraplanar, you might need to go to other planes to solve problems. You might explore other planes. Regardless, the Spelljamming mechanics give PCs another way to navigate to other places outside of spells and portals. Often, PCs will find that moving around the PMP becomes trivial around 9th level. However, being able to navigate and move large amounts of material around between planes does not become trivial - and that is where Spelljamming often comes into play. To me, the sweet spot for Spelljamming is between levels 9 and and 13 - after teleportation makes traveling the material plane mundane, and before plane shift makes planar travel easy.
3.) Spelljammer is more than ships: There are a lot of lore elements of Spelljammer that are unique and different. It feels like something unique when you really dig into it, or when you develop your own version like I have. There are major elements of my campaign that are highly influencd by the way I use Spelljamming, and they are amongst the most iconic elements of my homebrew. I have distinct factions, persaonlities, and long storylines that are based around Spelljamming and the influence it has on the setting.
If you're just running adventures or adventure paths, then Spelljammer probably is not that necessary unless you want to run a Spelljammer campaign. However, if ou develop a campaign world that is full of homebrew personality, Spelljamming is a great tool for mid to high level adventuring that can bring a distinct feel and isolate a 'new phase' of the game