Side-b is a really powerful option in the neutral game. It goes through a lot of projectiles and decayed aerials (which is pretty sweet by the way, I love not getting stunned by Falco's weak d-air). If you use it to replace aerial approaches, it's kind of difficult to counter because typical defensive options (shield or dash dance) don't really work. You can also use it as a universal punisher; imagine you react to a tech, but wouldn't get there in time, and if you try to grab they might spotdodge. One pretty cool option is to shellshift away, but you could also just side-b as soon as you're close to them and risk very little.
Not saying that I think side-b is overpowered or centralizing; Squirtle has a lot of really good moves in this version (side-b, bubble, d-smash, u-smash, b-air). But it's foolish to downplay how useful it is.