In Europe it`s popular decision to spend less money. At first it concerns to old cars with big engines.
If you have, for example, e34 535iA. You want to use this youngtimer everyday and you don`t want to pay for 18-20liters/100km - LPG it`s good choice to pay less. 1 liter LPG usually costs 30-45% from Petrol price (1 liter).
Engine starts on petrol and after heating runs on LPG.
End else - LPG is more ecological friendly
P.S. LPG = Liquefied Petroleum Gas
Let`s talk about this alternative fuel! ;-)