I use them to take out the big multi wound stuff. Other vehicles, carnifexes, daemon princes/primarchs, etc. although this weekend I caught an opponent in bad deployment. His warboss wasn’t screened at all and I was able to get first blood and warlord with one of the Reapers.
They force my opponent to do invul instead of regular saves - and most other vehicles don’t have invul’s. I always deploy them out of sight and try to keep them till the end of deployment. If going against a deep striking army or one that can cross the board quickly, I’ll screen with cheapo Warriors.
Kabal of the Green Hair