Objectively Russia may have nothing to gain from short military intrusions into neighboring countries, but these decisions are driven by more indirect goals.

There is plenty of evidence that Russia is actively testing the military readiness of countries close to their borders. For example, intrusions by Russian military airplanes into Finnish airspace have increased tenfold compared to previous years.

Dismissing the submarine reports as a NATO ploy looks like a conspiracy theory when it's clear that Russia is actively doing this stuff all over their European border.