It will do everything, that the old way of generating a netlist and importing it into pcbnew did before. It is pushing the netlist information directly without generating the file.
However, there are differences in the two dialog options, in terms of Check Boxes and Buttons, as noted below.
If “Load Netlist” is redundant, why is the icon still there?
Radio Group
Keep existing symbol to footprint associations
Re-associate footprints by reference
Check Boxes
Update footprints Delete tracks shorting multiple nets //Missing from "Update PCB from schematic"
Delete extra footprints
Delete single-pad nets
Buttons
Test Footprints Rebuild Ratsnest //Missing from "Update PCB from schematic"
Basically loading netlist to pcbnew is needed because someone may want to write a netlist by other means and read it to KiCad. It’s not redundant. Likewise someone may want to write a netlist from eeschema and use it for other purposes than just reading to pcbnew.
I think the “Delete tracks […]” option should be removed because it belongs to Edit->Cleanup and is there already. “Rebuilt Ratsnest” should be unnecessary altogether; IMO the ratsnest should always reflect the current situation on the board automatically (“should” means “must” in this context).
Maybe the developers just haven’t given any though for this because it works as expected and is secondary in importance. Those two are just small usability/UI design issues.
If you want to help, you can report those issues (separately, two reports) and see what the developers say.
someone may want to write a netlist by other means
That is an interesting thought
may want to write a netlist from eeschema
Typically to a SPICE simulator?
In 5.99 the icon has been removed from the toolbar
Hmm… It was a useful time-saver to import directly, without bothering to go back to Eeschema, then click buttons to write to file. I do not do anything else with the netlist file.