Window bars are almost always installed in order to prevent potential intruders from entering a house. It is true that this feature can prevent people from entering, but they could also prevent the occupant of the home from being able to exit in case of a fire. One case can be called to mind from the Philippines, where a number of people on the sixth floor of a hotel that was on fire, died due to not being able to exit because of window bars.
Another factor is that these bars could prevent egress if an intruder was already in the house and had entered from another point other than the window. Window bars could also decrease the value of the home while also giving the concept that the neighborhood is dangerous.
Having said all of these things though, the fact remains that in some cases it is a good idea to leave them in the house. In these cases, the first floor is usually the only floor with this need. It is a requirement to have a release mechanism installed in any window with bars in case emergency egress is necessary. These are some points to think about regarding window bars.