jQuery was important not just because of the feature set, but because it completely dominated the industry. jQuery and its ecosystem of plugins created a single community for JS developers. A lot of subsequent work wouldn't have been as successful without that strong community in place to spread ideas.
The other thing it did well was change the thinking about how JavaScript should interact with the DOM. In the past you'd have libraries that made DOM access easier, but they largely followed the same conventions as the underling APIs. jQuery broke from that and completely re-envisioned the API layer. The underlying APIs were treated as an implementation detail; this approach is something that subsequent libraries have adopted too.
The other thing it did well was change the thinking about how JavaScript should interact with the DOM. In the past you'd have libraries that made DOM access easier, but they largely followed the same conventions as the underling APIs. jQuery broke from that and completely re-envisioned the API layer. The underlying APIs were treated as an implementation detail; this approach is something that subsequent libraries have adopted too.