What will you do if you are injured at work or in an accident? Or, if you are doing hard physical work - what will you do when your body can no longer keep up?
Reading about your own industry or one that you might like to transition into can keep you abreast of needs or opportunities that you might be able to fill. It can keep you one step ahead of your colleagues (or boss). This can really give you an edge over another candidate, even if you are making a lateral change.
If you are not working or making a career change, this can be crucial! By reading about your chosen industry on a regular basis you will keep your knowledge current and be prepared for any opportunities that might present themselves.
Not only that (as if it weren't enough!) it can make you feel connected and confident - something you miss when you aren't working or facing a career change.
In addition to reading something about your industry I also recommend that you read about job searching or career planning. If you are unemployed, set aside an hour a day for reading career planning or industry-related information.
There are plenty of free resources on the internet, but don't forget about your local library for books and trade magazines.