Venues tend not to sell 100% of tickets precisely because until the stage set is built and in place they can't be sure which seats will have their views restricted.
Once the stage is in place in the venue they can then check and see. Often they find they can release tickets for sale at this stage (it is worth phoning a venue before a show if you want to go but it is "sold out" through normal outlets for this reason). Alternatively, it means they have spare capacity to move people should places they thought would be ok turn out to be restricted after all.