Helicopters are used because they have three main strengths. These are stealth, standoff, and cover. First, let's talk about stealth. During the 1990 Gulf War, helicopters actually fired the first shots. Their mission was to destroy the early warning radar in Iraq. They were able to successfully destroy the radar while maintaining stealth. The Iraqi radars were in search mode for the whole mission, which meant they went undetected.
A helicopter has the unique ability to fly nap of the earth better than any other aircraft. Nap of the earth, means to maintain near zero altitude with respect to the ground-basically to become one with the terrain. A fixed wing aircraft would move too fast to follow every contour of the terrain. A helicopter can go slow enough to hug the terrain.
The second thing a helicopter has is the ability to maintain standoff. It can hover or go slow, instead of charging in at 350 to 400 miles per hour, like a fixed wing aircraft would do. The third thing a helicopter can do well is use cover. It can maintain standoff range while using the cover of a terrain feature. It can hover and move up and down behind a terrain feature.
The method I like to use to maintain stand off range behind cover is to circle it and dive under it. By this I mean that I approach it, turn 90 degrees so that I'm going sideways with it and then dive towards the ground. The reason I like this approach is that you can climb and descend faster why you're moving then when you're hovering. When you are hovering behind cover, remaining stationery reduces your ability to climb and descend quickly.
Also, I have noticed that you can avoid missiles better using a terrain feature by turning 90 degrees away from it and diving into the ground. Moving sideways with respect to the terrain feature while diving is more effective at getting the missile to run into the terrain feature than to be head on with it while diving. What I mean by being head-on with the missile is facing it, moving towards the terrain and then diving into it. It is better to create some sideways motion with respect to the terrain by approaching a terrain feature, turning 90 degrees just before you reach it, and then descending below it quickly.
The weaknesses of helicopters are that they tend to easy targets at high altitude. They are too slow to evade ground fire through maneuvering- and just too slow to be hard to hit. They need to use the terrain, just like any ground vehicle. In fact, another name for helicopters is the air cavalry, because they are similar to ground vehicles.
They are not inherently less safe than fixed wing aircraft. They just have different dynamics, like a ground vehicle. The helicopter needs to have situational awareness of the position of the enemy before advancing. This is unlike an aircraft, that can charge forward and use the high altitude to create standoff between him and the enemy ground units.
This is to not say, that a helicopter can not just move forward without pausing to look behind every terrain feature. Like a tank, a helicopter has a spotter. The co-pilot/gunner in the backseat of an Apache helicopter has the job of continually scanning the terrain for threats. Like the gunner in a tank, he will call out a threats as he finds it. The gunner in a tank will call something like tank left-front, which means he has just found a tank in the direction of the left front. Likewise, the co-pilot/gunner in the backseat of an Apache helicopter, will spot a threat and announce something like troops three o'clock 2 clicks. After the co-pilot/gunner calls out the threat, it is then displayed to the pilot on the tactical situation display. The tactical situation display will than display the direction and the distance of the threat.
What about missiles? Are not helicopters a sitting duck for guided missiles on flat terrain. Not exactly. Helicopters can avoid missiles mainly by running them into the ground. Running missiles into the ground does not literally mean making them crashing to the ground. It means making them have a hard time to avoid a dive that will run them into the ground.
How I avoid missiles is I put the missile by my three or nine o'clock, descend quickly by a steep dive, and then turn to put the missle at my 10-11 o'clock or my 1-2 o'clock. This act of putting the missile at three or nine o'clock, diving, and then turning partially gives the missile some turning problems. Also combined with countermeasures like jamming and chaff, the missile can not make the turn into you and hit you.
An interesting side note is that helicopters, according to the DCS(Digital Combat Simulator) Black shark simulation, can avoid missiles purely by maneuver only. DCS Black Shark is a simulator of the Russian KA-50 Black Shark attack helicopter. The Black Shark has two coaxial rotors and no tail rotor. By purely by maneuver, I mean the helicopter can be a decent altitude above the ground while doing a maneuver. According to the simulator, missiles can be avoided through a barrel roll at a good distance above the ground.