Firmware zip basically contain latest drivers of cellular modem and drivers that improve stability and battery life of you phone.
Custom ROM's like Cyanogenmod may miss these key updates.
Ask yourself have you ever felt trouble with cellular signal and less battery backup than before or only toggling in and out of airplane mode to restart the connection probably the reason can be missing firmware [baseband] Drivers.
As we know updating firmware file doesn't touch any file on system or mess with your data so it's safe to flash but make sure it's for your device.
Custom ROM's like Cyanogenmod may miss these key updates.
Ask yourself have you ever felt trouble with cellular signal and less battery backup than before or only toggling in and out of airplane mode to restart the connection probably the reason can be missing firmware [baseband] Drivers.
As we know updating firmware file doesn't touch any file on system or mess with your data so it's safe to flash but make sure it's for your device.
Just searched on google and gathered information to give everyone idea why developers of custom rom are mentioning to flash firmware zip on redmi 1s.
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