Something interesting about helicopters that most people are unware of is how little they actually enjoy hovering. Its sort of the whole reason to use a helicopter over an airplane, for its ability to fly very low and slow and takeoff/land in confined areas. But vertical flight is actually remarkably inefficient and the reason for it has to do with aerodynamics.
An airplane or helicopter uses a particular shape in its wings or rotors known as an airfoil to generate lift.
To put it simply, airfoils generate lift as a function of airspeed (how fast the air is traveling over it) and angle of attack (AoA, the angle that the airfoil is traveling through the air). Airplanes travel forward and adjust their speed and pitch angle (nose up or down) to change the amount of lift whereas a helicopter spins its rotors around at a constant speed and adjusts the blade angle to change the amount of lift. But there is another factor at play with a helicopter - induced flow.
Induced flow is the vertical component of airflow through a helicopter's rotors. This is the air getting sucked down into the blades from above and being expelled below. It is strongest during a hover and at low airspeeds.
Because the airfoils of the rotors are traveling "forward" from their perspective they experience airflow in the opposite direction of rotation (the rotational relative wind). And because of the induced flow, these two components combine to make the oncoming airflow into the rotors at a slightly downward diagonal angle.
This diagonal "resultant relative wind" effectively reduces the rotor blade's angle of attack and the amount of lift it can generate. So an even higher angle of attack must be used to compensate for the loss of lift which has a penalty of more engine power required. This is why it requires more power to hover than to fly with speed.
As the helicopter accelerates into forward flight the downward induced flow passing through the rotors gradually transitions from a downward vertical angle, to a diagonal angle, and finally to a nearly horizontal angle.
Now there is much less induced flow and the rotor blades have a much larger angle of attack, generating significantly more lift. This requires less engine power and an increase in aircraft performance.
Here are a few screenshots from Microsoft Flight Simulator which does a decent job representing this phenomenon to demonstrate the performance increase.
In the first set the helicopter is hovering at standard temperature and pressure at 2500lbs gross weight near sea level. On the left side there are eight smaller gauges in two columns of four. The top right of these instruments (right below the highlighted blue square) is the torque meter which provides an indirect indication of engine power. Its currently at about 58% to hover just a few feet above the ground.
This next set was taken after accelerating to forward flight. I've kept the power at the same 58% torque and the result is now the helicopter cruising at a steady 60 knots and a 500 foot per minute climb rate.
A bit of speed made that much difference with the exact same power setting used to hover just a few feet above the ground.
This is why helicopters often take off like an airplane whenever possible. Accelerating from a hover over a short distance before climbing away.
In many cases they cannot takeoff otherwise. Atmospheric conditions, weight, and power restrictions can keep it from climbing vertically. This means that a pilot has to carefully plan departures and approaches to confined areas to prevent getting stuck. The opposite can be true for landing. You may not have the power to hover high over the landing spot so the helicopter has to approach with speed and a shallow angle like an airplane. Carefully controlling descent rate and gradually bleeding off airspeed to arrive right to a low hover over the spot.
