Most importantly you can protect your brand.  You wouldn't want your competition grabbing your "name" and using it to drive traffic intended for you to their site.  Further, you certainly don't want to tarnish your brand someone launching an offensive themed website on the .net version of your domain name.  You also would not want to decide at a later date to register the .net version and find a speculator now owns it and will happy to sell it at a substantially higher price.

Another good reason is to separate segments of your website.  Sometimes this can be important for security, convenience, or search engine placement.  If you run a log in customer area for collaboration or uploading/downloading of files you might want to separate and isolate these more exposed segments on a different domain space like the .net version of a .com domain.  This can also be helpful if you need or want to separate a website's front end from a third party ecommerce platform you may be using.

While registering both a .com and .net version of your domain doubles your domain investment, domains are very inexpensive when compared to the value of your brand.  This may be the best investment you can make for your internet and web presence, or perhaps the most expensive brand related mistake you'll make.