There's a vast world of difference between "sandbox games" and the type of open-world games that Insomniac makes. For a series like say, Fallout or Grand Theft Auto, the excitement comes from the opportunities the game offers you. Kill every cop you see, or set of a nuclear bomb, or whatever. What makes Spider-Man so much different than other open-world, explore and do anything games is that simply the act of traversing that world is just as fun as the opportunities and story that is being told. Insomniac has gotten in the business of making games that are simply fun to just walk around in.
All the way back to Spyro and the Ratchet & Clank series, up until Sunset Overdrive, Insomniac has been great at making games that are fun to just hang out in. Grinding around the city in Sunset Overdrive, or bashing enemies in Ratchet & Clank is just as great as the story - if not even better. Almost every time I have finished the story of an Insomniac game, I have wanted to continue the game, exploring the world and just maneuvering around the worlds that are insanely fun just to move around in.
Spider-Man might be the pinnacle of Insomniac's ability to craft massive worlds that are fun to move around in. In many ways, Spider-Man owes much of its design to Sunset Overdrive, a vastly underrated game from Insomniac that does many of the things that Spider-Man does so well. One of my favorite things that Insomniac does with these large scale games is pretty early on present all the things you can do in the world - and importantly, WHERE you can do these things - and allows you to discover the world through these side missions. So many games will say "oh, go find 100 of these items, good luck!" but thankfully, Insomniac always lets you know where these items are. This makes the game simply fun, instead of borderline stressful, and especially with photo op missions and the like, will allow you to see all the different fantastic elements they've recreated in their version of New York City.
Spider-Man also borrows heavily from the other standout superhero series: Rocksteady's Arkham series of Batman games. Not only are the various mission structures very similar, but the combat especially is straight from the world of Batman. Yet Insomniac improves upon this, making both the missions and the combat not quite as brutal as the Batman universe. Wisely, Insomniac has given this Spider-Man world a light touch that makes this game charming over challenging. But if the player wants the challenge, they've hidden plenty to seek out for hours of difficult missions.
But the key to what Insomniac does so right with this game isn't just in creating a game that is incredibly fun to explore and move around in. The key here is in the term "friendly, neighborhood Spider-Man." Peter Parker and Spider-Man are sources of purity and good in a city packed with maniacal villains and criminals on every corner. Parker here is a wonderful companion, doing everything he can to improve the life of those around him. He's always assisting Aunt May, giving Miles Morales a purpose and assisting MJ when she needs help. Yet the overall arc of Spider-Man presents the idea that even though Parker's ambition to be the savior for those around him, he needs to embrace the help that his friends can give him. This is such a well told Spider-Man story, and considering I've often found the story the weakest part of many Insomniac games, I think they really nailed the right tone that this game needs.
Insomniac's strength as a video game company has quickly become creating games that are equally enjoyable no matter what you're doing in them. Spider-Man literally throws you right into swinging down the streets, and almost 30 hours of gameplay later, doing that still has the same amount of pure joy as it did in the beginning. But every aspect here is fantastic, from fighting the newly formed Sinister Six, sneaking around an art gallery as MJ, or simply walking down the street and greeting your fans or taking photos of the Wakanda embassy. Insomniac excels at making a sandbox where every grain is worthwhile.