When it comes to landing gear, helicopters have two basic types: skids and wheels. Skid gear is always fixed and wheels can be fixed or retractable. Which one is better depends on the mission the helicopter is performing.
Skid landing gear is simple and lighter weight, so it is the best choice for small helicopters as weight is always a consideration. Also, skid landing gear needs very little maintenance, but the drawback is that ground handling is more difficult. In very small helicopters (such as the Robinson) ground handling wheels can be attached to the skids and the helicopter moved around by one person. Larger helicopters (such as the Bell 206 or AS350) can be moved around with ground-handling wheels, but it normally takes a couple of people. There are a variety of power-assisted tugs and transporters that can help someone move larger helicopters by themselves. Another method of ground handling that operators use is a platform dolly that the pilot can land on and then tug the dolly and helicopter around. However, pilots need to be comfortable with dolly landings as several accidents have happened from pilots misjudging the landing.
On larger more powerful twin-engine helicopters weight is not as big of a concern and retractable wheels make sense. Wheels are nice because a helicopter can ground taxi (as opposed to hover taxi) around other aircraft and people without worrying about producing a high downwash. Retracting the gear also reduces drag, allowing for a higher cruise speed. Attaching a tow bar to the nose wheel makes moving the helicopter around a lot easier as well.
The engineering decision to use retractable wheels or fixed skids is basically a trade off. The retractable wheels give better speed, ease of handling, and the ability to ground taxi. However, they are more complex, require more maintenance, and increase the weight.