My landline is still my primary phone. It is the only phone number I give out. My cell phone is my emergency phone. My wife, son, daughter and daughter in law are the only ones with that number.
But my primary reason for getting a cell phone back in 1990 was to have a second phone number that I did not give out. I worked graveyard shift and slept 9 am to 5 pm during the day. I got tired of getting woken up by phone calls during the day, so I got the cell phone. When I went to bed, I turned off the ringer on the landline, and put the volume on the answer machine to zero, and turn on the cell phone. That way I could sleep undisturbed, but still could be reached by those who truly might need to wake me up in an emergency. My bosses knew they could call me ANYTIME and leave a message and that I would return the call when I got up at 5 pm. They also knew they could call my wife at work and have her call me on the cell phone if there was a reason they needed to speak to me immediately. That never happened, leaving a message and waiting until I got up to call back always got everything handled.
My wife has a cell phone too, and the same situation, nobody gets that number, just the landline. And to be honest, a year into retirement we are considering getting rid of both cell phones and getting one pay as you go phone for travel as we are never apart anymore. The landline will remain our primary phone.