This may seem relatively obvious, but blogging about a hobby is a great way to blog. There are lots of people out there looking to try new things but often they have no idea where to start or where to find necessary information. Sometimes people are just looking for inspiration.
Whatever your hobby is: fishing, boating, beading, sewing, collecting - whatever - you can guarantee that there are others interested in what you do. They may just want to see a bit of your work or hear some of your story or they may want to learn the hobby for themselves. Whatever their interest, you can provide them with something of value.
Taking the time to blog about your hobby can also be a good way to keep track of your own personal progress. Over time details of particular events or projects can get a little hazy, and having a blog that you can refer to over time is a wonderful way to be able to travel back through time and see the neat things that you have done.