If You Have It And They Want It.

While having a reputation for being a dismal lot, economists have offered spectacular clarity to the business of running your law firm: the law of supply and demand.
Thus, if your law firm's knowledge and expertise with a high-impact problem is in high demand by clients and clients perceive that your expertise is in relatively limited supply, you have a price-fee advantage.
Simply put, if you have it and few other law firms have it and clients want it, then you have leverage to get paid twenty Indian elephants. On the flip side, if the said expertise applies to a mature matter you really haven't a price fee-advantage, where mature means that the problem is well understood by clients and other law firms and there is little risk of bungling no matter who handles it.

One reality that then affects your law firm as part of the sweet package of the law of supply and demand is: the issues that are right now limited to few law firms will be open to a whole lot more as other law firms acquire said expertise and clients understand the issues from regular exposure so your law firm needs to be searching for new issues.
It then does make sense for you to focus your law firm in areas where you can have the most impact i.e. where they ain't. Eric Silverman began in project financing in the 80's when in his entire law firm, only one partner was involved so Eric got to manage client relationships as well. Fast forward to 20 years later and project financing contributes ¼ of the entire law firm revenues.
The point for your law firm is that you will always need to compete and sometimes that will mean leaving the majority and going to empty places you can dominate. It helps if the laws of competition (namely, you don't have the resources to go head-to-head with the bigger law firms) compel you to develop neglected areas of practice.