Mainly because it's good for you while being enjoyable and satisfying. Many people need some sort of motivation to get exercise, and jogging and weight circuits aren't very appealing. Walking is recognised as both great exercise and low-risk, you're much less likely to injure yourself, particularly when you start out. If you take up jogging, you need expensive footwear to avoid damaging yourself, and you can't spend as much time taking in the scenery.
Walking is much less likely to injure you, and allows time to look at the view, smell the flowers and (horror?) talk to your family. The best exercise for weight loss is said to be brisk walking to the point at which you can still hold a conversation, for 60 to 90 minutes at a time. You may want to work up to "brisk".
So now you need to go and buy some gear for bushwalking? Worried about the expense of all that fluorescent polyester stuff in the shops? Me too! Certainly to start with it's unnecessary. The largest expense for your first few walks should be the petrol to get to the start in your car. Share that with some friends too if possible!
You do need some items with you for basic safety and comfort. Firstly, on any walk where it's recommended, do not go out without a jumper and a raincoat. The jumper should be wool, and at least moderately warm. Many people have polyester fleece clothes these days, and these are even better. Don't rely on a fleecy cotton top, once wet these are useless at best, dangerous at worst. Check the label. You probably own a good woollen jumper. If not, in fact you can get them at opp shops.
The raincoat needs to be waterproof, preferably with a hood. Don't rely on spray jackets or quilted cotton/nylon coats which will let you get wet after only a few minutes of decent rain. Japaras are fine if in good repair. Don't get an expensive breathable Gore-Tex coat to start with - the $300-$500 these cost will be overkill if you don't go on to expand your walking horizons. However, breathable coats allow the moisture (from your "glow") to escape through the material without allowing rainwater in, improving comfort.
How fit do you need to be?
Not very, to start with - this is something you can work up to, and if you walk regularly you'll see an improvement in your ability.
Will it hurt?
Unfortunately, depending on how hard you push yourself, and how little exercise you've done before, it's possible. Don't overdo it, listen to your body (and your doctor if necessary) and keep the pain to a minimum. After all though, take heart from the achievements which led to the ache.
How fit can you get?
For some people the sky's the limit, and eventually they'll want to conquer Everest or walk the Great Dividing Range equipped with a water bottle and a compass. However, most of us set more local and achievable targets. If your aim is to walk to South Cape Bay (14km return and flat) that's fine. Perhaps you would like to climb Mt Anne, or do the Overland Track - great, just work up to it!
If taking up exercise, and aged over about 40 (or perhaps younger) apparently you should consult your doctor about any exercise regime. Walking is certainly safer in this regard than other activities, but we've all heard of the people who collapse while climbing Uluru. That's not a big climb compared to some in the Huon, although it's likely to be cooler here. So take advice, particularly if you have been getting no exercise at all, or if like me you have a tendency to store sustenance comfortably about your person during periods of inactivity in preparedness for more strenuous times.
A walking club can be a great way to get into walking. There's always something on, and you can always find someone to walk with. Generally they know what they're doing, and the leaders can be relied upon to know how to keep people safe. The Hobart Walking Club is one place to look, or try here.
The Hobart Walking Club have been around forever and have something like 1500 members. They organise many walks each week and there are multiple walks to choose from every weekend. There are mid-week walks as well for those blessed with availability. They take "prospective members" who have to go on enough walks to qualify for membership. They are well known for introducing people to walking, and looking after them quite well.