Question about Limited Mods

So my brother has just finished a limited mod and it has sparked my interest so I have been looking around at cars. On thing I have noticed is some cars looks different then others. I assume due to upgrades, years , different builders so forth and so on. One thing I cant figure out is why some cars have what looks to be a safety bar from just behind the front tire and ending about midway up the car right behind the driver compartment. I'm sure the reason for it is safety but do different manufactures hide that bar behind the body or leave it out all together? Is there something to be said about the kinda of chassis it is and the quality? Should I stay away from cars of this kind or is it not something to worry about? If I do get a car with this bar can I hide it behind the body of the car? Any help trying to understand why some have it and some don't (either by it being hid behind body or not there) would be awesome.