well i keep a few in my bag just in case. and i know the US military uses them as well as most law inforcement
for cheap hand cuffs .
i use them for more permanent aplications where i would not want to waste para cord. like long term survival structures,equipment to equipment attachments,keeping para cord/rope/twine bundled and redy to dispence(then just tighten the zip tie after you take some cord) on the out side of a pack or belt, applications that need some thing a bit more resistent to abration. i know there are alot more i just cant think of em now. but mostly its cuz i dont want to cut up my para cord. but i dont carry them every day in my psk as its very small and almost non existent