I think the biggest benefits of *A Miles and More are:
Companion fare for 50% of the miles needed
Ability to pay extra miles for a redemption ticket and get a seat if one is available, even the last one going and if all the redemption seats have already been allocated.
The ability again for extra miles, to fly the route in any order once the ticket has been booked and issued.
Senators receive priority in getting seats for miles. I got 2 F seats with Swiss from zrh-mxp just seven days before the flight.
A genuinely helpful staff manning the 24 hour help line.
I’m sure I’ll think of a few more benefits later!