Wearing a mask 'may' slow the spread of Covid-19 but that's it. We've been wearing masks....we've been social distancing...we've been washing our hands...
How does wearing a mask create or contribute to herd immunity? It doesn't. What we need to beat Covid-19 is herd immunity, which can come about naturally or via a viable vaccine.
If wearing masks slows the virus, then it slows spread just like herd immunity (while also slowing the acheivement of herd immunity). If masks do nothing, then they don't slow the achievement of herd immunity and may speed it up. Either way, masks don't hurt even if they are not helpful.