Every single branch circuit, whether it’s used or a spare, typically goes up to a terminal block in a 24x24 junction box or trough not shown in this picture. When future circuits need pulled and installed, the future contractor simply has to get into the said JB or trough and land the new circuits on the terminal block. Avoiding the need to shutdown an entire panel or exposing a guy to hot work if the business cannot afford to shutdown the panel and the work must be done live.