A landscape gardener will have more specialist skills than your everyday gardener. They will be able to design and craft a garden that looks entirely new, ensuring that it will be designed on a different scale and will look far more impressive than it does at the moment. By this token, people tend to only hire a landscape gardener if they have a big project on. This might include levelling out areas of the garden, planting up large areas, mowing extensive lawns, creating water features or ponds, tree surgery, and creating overall designs for the garden layout.
If you think you might want a landscaper to take a look at your garden, see if you can find someone local who will be willing to have a look. They can then give you a quote, so you can decide if this is reasonable for the amount of work you want doing.