It's cheap, thus plentiful. It is relatively easy to train a large number of crew on. Not everyone will be a "marksman" with it, but a lot of folks can at least step up, load it, and start firing in a pinch with some simple training. It is small...it can be mounted in many locations. Larger mounts need space and weight (and more intensive training, and money.) It is literally better than nothing.

It is part of a layered defense on our "warships." Those have either a 30mm gun system, or more commonly, the MK 38 25mm gun system...3 on carriers or the LHD/A, 2 on the rest. In the event of a swarm attack, those 2 or 3 systems will be overwhelmed. The .50 calibers and M240s then provide the next layers, as things get closer. And since .50s and M240s can be placed more numerously in more locations, they actually cover more area. (I found a Youtube video titled ".50 calibers on carriers." Expecting/hoping for narration, I instead got 9 minutes of multiple mounts being loaded and fired. However, the backgrounds showed all the shots were indeed from carriers, and they were "everywhere." That showed me they have unmatched coverage! Looking at the 3 MK 38s, they have all kinds of blind spots.)

Our warships (CVN, CG, DDG, LHD, LHA, LSD, LPD, LCS) all have heavier gun systems (as well as some missile systems like RAM and even Hellfire.) For our auxiliaries, the .50 caliber and M240 are cheap, and "sufficient for peacetime security needs." Up-arming them for war takes money, and that requires a good deal of resolve/commitment. (WWII was an "all-in" war. There was little debate about it, and "the nation," to include Congress, were willing to drop everything and pour money into war production. Nowdays, we expect to fight wars "on the side." Their budgets then compete. Auxiliaires are low priority in all that.)