I used to run prior to buying my Garmin 305, and I even ran a 10k with decent timing, after deciding last year to leave the couch behind. I had a myriad of foot related injuries and at one point my sports med pretty much told me that all lower extremities will require replacement. I hope he was joking. But the fact was I had no clue as to how I was pushing myself. Even the course around my house I knew as if the back of my hand, I didnt know whether I was doing better one day vs the previous, whether I am pushing myself too hard, how far I was running etc. I would drive my car around the course usually, If I ran different to calculate the mileage, but that became a hassle (found out later that there are other ways such as Mapmyrun which overlays google maps etc.)
My two bit advice to you would be dont buy a gps training device (which is what it is) unless you were training for something. And something bigger than a 5k or a 10k. If you just started running, then run for the fun of it and when you have got that in your blood, get a training device, when you are ready to step up to the next level. A gps device the first time you start running would overwhelm you with all the data (and Garmin 305 buries you with it, and I love it!). You need all the data when you are ready to make sense with it. Initially, you should smell the crisp air outside (or the smog), feel your heart pounding inside the ribcage, see the next hill as you race towards it and its more gratifying than a lot other things, like reaching for the next bag of chips.
A Garmin 305 with its heart monitor will give you tons of data. It will poll your position every 3 seconds, and you can use a tool like SportTracks to overlay that on google maps or Google Earth to see what you burned through. Garmin has its own training tool, like Garmin Connect, which previously sucked, but now is much better. Still I would like to direct you at Sport Tracks as its free and gives you a cumulative representation of your training than other tools. There is nothing better than seeing a month worth of data and see that you have ran 100 miles in the last one month, which days you ran, what your average pace were, your splits/laps. And oh..and graphs, more and more graphs. You can also track as to what parts of the course you were running fast vs slow, your heartbeat zones and the areas of the course where you were about to pop so that you can be better prepared etc. The Garmin 305 does a piss poor job at calculating the calories burned, as it computes it based on the distance covered, not on your heart beat which is a better route. But as long as you burn more than you take in, even if its a rough figure, you would lose weight gradually.
Sure, you dont need Garmin 305 (which is rather bulky, but once its on your hand you dont feel its there) or any other training devices unless you are prepared to take your training to the next level. I am running a half marathon in November and I am treating my training just as I would treat anything else thats important in my life. I have a goal of a set number of hours:minutes before I cross the line and I am not ready to leave that to speculation. I train because I want to be injury free and better prepared. And thats what I have my Garmin. YMMV.