There are various reasons why you should use a treadmill for walking as why you should walk outside. The best would be to use both, that way you will never have an excuse to not walk or exercise.

Why you should walk outside:
*Fresh air makes you alert and energetic and is great for your lungs.
* You will be close to nature and can relax in a different manner than if you are walking in a gym full of music and noise.
* It's a good time to get together with a friend and enjoy a walk outside. Again, an outside environment is also more relaxing and will make it easier to talk than if you were in a gym.
*You can enjoy high hills and get a good leg workout.
* You can vary your routes and always see something new.

Why you should use a treadmill:
* You don't have to worry about bad weather as you will always be inside.
* You can set the incline and are not dependent on hills. Although that is not the same thing as walking up a hill for example but will increase the resistance and making your workout a bit more "difficult".
* You may find you have walked further than you would have had you walked outside. If walking and watching TV at the same time you can get so into what you are watching that you will not notice how far you have walked.
* It's easier to know how far you have walked than if you walk without a pedometer.

As you see it can be a good idea to combine the two ways to walk. You can then make sure you always get the workout you need without having to stress yourself over it.

Combining outside walking with a gym membership is great but if you feel you don't want to join a gym then you might want to invest in buying a treadmill for your home. Make sure you do use it though and don't let it dust in a corner somewhere.