How is knowing watts better? It’s clearly much worse if you want to ride at a similar speed. The W/kg for a 50 and 100kg rider going at (say) 38kph is much closer than their watts. Even on level ground.
If you are super-light or super-heavy then you should realise that your W/kg is likely to be a little higher or lower than the quoted figure, assuming the pack follows the guidance reasonably closely. But if everyone rode at (say) 200W then they would be riding at very different speeds.
BestBikeSplit has these times for various weights/watts - you can see heavier riders always have a w/kg advantage whatever the gradient:
Tempus Fugit:
300w/200lbs/3.31w/kg - 25.54mins
149w/100lbs/3.29w/kg - 32:15mins
149w/200lbs/1.65w/kg - 33:42mins
Ven-Top:
300w/200lbs/3.31w/kg - 1:31.38hrs
149w/100lbs/3.27w/kg - 1:41.27hrs
149w/200lbs/1.65w/kg - 2:58.00hrs
Does BBS copy zwift physics where your weight is used to change your CdA quite substantially?
I think this is where the widespread myth comes from - many cycling physics simulators allow for the separate specification of weight and cross-sectional area, and changing weight alone makes very little difference to speed at a fixed power (on flat road). In zwift, they are directly linked. Zwift’s general approach seems reasonable to me but I know that others complain that the influence of height and weight is too great.
I don’t know how the modelling is done though would be interesting to see how well it fits in with what people experience.
Here’s similar from Bikecalculator for real life riding - no account for height/shape I assume:
Flat:
300w/100kg/8kg = 36.25kph
150w/50kg/8kg = 29.08kph
150w/100kg/8kg = 27.57kph
8%:
300w/100kg/8kg = 11.14kph
150w/50kg/8kg = 10.23kph
150w/100kg/8kg = 5.67kph