Fire fighting is a complicated and delicate process, involving complex scientific tools and a thorough understanding of the various kinds of fire, their causes and how best to attack them. It's not as simple as throwing water at the flames with a hose and hoping for the best, and doing so can actually cause more damage than the fire will if left to its own devices; that's why using the correct fire fighting equipment is so important.
Dangerous Structural Damage
Each building's structure is different, depending on what it was constructed for and when. A 15-story apartment complex necessarily has thicker, sturdier walls than a wooden farmhouse from the turn of the century.
If fire is eating away at a structure, using too forceful of water pressure can further weaken the walls and collapse the structure partly or entirely, causing massive property damage and possibly posing a risk to anyone still trapped inside, as well as those nearby outside.
Harm People Stuck Inside
Certain firefighting tools use oxygen-gobbling chemicals to suck the air from an enclosed environment. While this is an innovative and intelligent way to fight fire, if used improperly (in extremely tight quarters, for example) it can harm those stuck inside the building. The chemical itself is nontoxic but will rob survivors of what fresh air there is available and cause an unnecessary loss of life.
Likewise, simple fire hoses come in various power ratings according to how high, how far and how strong a water stream is required. If a fire hose is set too high for a smaller fire, it can easily cause massive damage to anyone in the way, such as survivors inside.
Conversely, if the equipment chosen is not a high-enough strength or effectiveness, it can severely impair the firefighter's ability to do his job properly and ultimately allow the fire to cause more damage than it may have if properly countered.
For example, a hose or a firetruck incapable of creating a pressure rated for fires above four stories is not effective in the event of such an emergency. Likewise, a single fire engine, regardless of its pressure rating, cannot deal with a multiple-target fire emergency where firefighters must deal with more than one blaze simultaneously.