The issue of harnesses is a slightly tricky one. Owners use a harness and leash to allow their cats to enjoy the outdoors without roaming too far or straying onto roads. So far, so good.
A Maine Coon looking smart in a harness.
However, many cats find the harness a stressful experience, and at the very best they will feel a bit annoyed that their roaming instincts are being hindered. The leash (and the harness too) can become tangled too, if the cat is unsupervised.
If your cat seems unconcerned by the extra hardware, that's fine. If she does look stressed, try to reassure her. She will probably get used to the harness in time; but if she doesn't, you will only make her unhappy if you keep attaching her to it.
The best way around this is to get a cat that doesn’t want to stray too far in the first place: see the Indoor or Outdoor Cat section of this guide for some good choices.