Well, in the instances of Gibraltar and the Strait of Marmara, if a power has ships their and another country they're at war with tries to pass, there is an automatic interception, as opposed to the lesser percentage chances in other naval regions. There are, of course, more sea territories where this holds true (The Skaggerack, for instance), but there is a benefit to holding these territories as national waters. A lack of attrition means you can camp a massive navy in this territory and block any enemy traffic in or out of the Black Sea (Marmara), the Mediterranean Sea (Gibraltar), or the Baltic Sea (Skaggerack)