trx output power config in bts

Dear team could you pls help me with this question   In bts config  we have following entries:   trx 0    nominal power 23    ! to use full TRX power, set max_power_red 0    max_power_red 20   and   osmotrx tx-attenuation (oml|<0-50>)     So why do we need these «max_power_red 20» and «osmotrx tx-attenuation» which both as i understand weaken the signal? If we want to have a weak signal why we just don’t use only «nominal power» with whatever weak or strong signal value we want?   -- Mario Lucas