Trademark registration has the direct effect of giving an owner offensive powers. More specifically, the owner is given the right to sue an alleged infringer in order to either stop the infringer from using the mark (i.e. injunctive relief) or collect damages for harm related to the infringing use. An owner can also block (through opposition proceedings) or cancel (through cancellation proceedings) future registrations of confusingly similar marks.
One thing that brewery owners may overlook is the fact that owning a federally registered trademark also, indirectly, gives the owner defensive strength. For instance, a potential plaintiff is less likely to sue a brewery for infringing its mark when that brewery holds title to a registered trademark for that mark. Also, another brewery will not be successful at filing its own trademark application for the same mark your brewery has been using for years.
Registered trademarks are, in my opinion, even more important within the beer industry than in most other product-centered industries. Consumers tend to be very loyal to their favorite beer brand. The brand tells the consumer what type of quality to expect from the beer they purchase. Trademark law provides breweries an opportunity to develop their brand without the fear that other brewers will come along and sell beer in a manner that consumers will associate with your brands.