HARMS generally are not for winning navy, but they are for buying time. They aren't only useful against subs, but can also delay ships.
Depending on the situation, you might buy enough time for your team to make torpedo planes so you can get back into the water. Or you just delay the inevitable invasion on one side of the map. Often in Seton's there is a "race" going on between top and bottom ponds. If one team is winning up top and the other is winning in the bottom, that doesn't mean it's even. If the top team is faster to do shore bombardment (blow up mass extractors) while the bottom team slows things down by making HARMS, the bottom team has a better chance to win the game.
Bottom team gets to keep more mexes, has more eco, and can spend it on different things like nuke launchers or scathis or hover spam. Timing and speed are hugely important in strategy games. It's easy to overlook that if you're only focused on who wins and who loses in a lane.
And it's the same thing for slowing down the land invasion (usually this looks like: the enemy team builds factories on your shore and starts spamming light artillery)