Where a proper P&ID software differs from a drawing tool like Visio or Diag is that the P&ID tool can output a parts list listing all components and connectors. Components and connecting lines can have additional specifications (like the power and type of a motor, or the size and material and pressure level of a pipe). This specifications can then be written to the material/component list.
Confession time: I've never worked with a P&ID software, this is just what I think (from poring over plans and counting valves) would be really helpful.
What core features do working engineers need from a modern P&ID tool?
Edit to clairfy: I'm not looking for a specific product, I'm looking for the core features that make a tool great.