Runner's Companion goes into detail on what each level of Lifestyle entails, including numbers for how hard it is to break into the place based on the Security rating.
Also keep in mind that if they're really skimping on Lifestyle, then they may have to pay out of pocket or scrounge for things that a character with a better Lifestyle could just consider "taken care of." Fuel for vehicles, hotel rooms, meals at nice restaurants (or any restaurant for Squatters, or any decent food, or food period for Street Level), Matrix access, normal clothing, shoes, something to sleep on besides some cardboard on the ground in an alley....
Lifestyle is great because it saves players from having to worry about tracking payment all those stupid little things like food and lodging that aren't related to shadowrunning. If they don't take advantage of that by buying at least a Low, then they have only themselves to blame when they have to waste time and money on mundane crap.