These processing nodes can be queried and joined on (with SQL) at any time to easily power realtime dashboards and other analytical applications. Check out the original announcement [0] and the Stride docs [1] for more detail.
And to answer your question, no, Stride is not simply a frontend for PipelineDB. While it does use PipelineDB and PipelineDB Cluster extensively, it also uses other systems to provide unique capabilities, and all of this is ultimately synergized behind a dead-simple HTTP API.
We've found that a hosted database isn't actually all that interesting for users nowadays, and near impossible to build a business around because they've essentially become a cheap commodity. So we aim to deliver maximum value to users by nailing one use case (realtime analytics) with a highly focused API that eliminates most of the complexity and decision points you'd encounter when trying to do the same with a generic database.
[0] https://www.pipelinedb.com/blog/announcing-stride-a-realtime...