you can use them for "free" (ha!) flights
or to pay for accessing KLM lounge at Schiphol, or to pay for a carton of Marlboro (or anything else allowed) on a Swiss flight, or....anything else you can read on an inflight magazine
I think the only sensible approach nowadays is to be a member of one airline's loyalty scheme in each of the three big alliances
+1
Ciao,
Ulxima