Tobias Oder has this interesting analysis of Russia’s sea denial capabilities in the Baltic Sea on cimsec.org. Essentially, the point is that they can close the Eastern part of the Baltic Sea quite effectively, should they want to.
Oder’s analysis is written from a German perspective, but I think many of his recommendations could be broadened out to other countries. They include the attainment of a strike missile capability to challenge Russian surveillance and launch stations, more advanced electronic countermeasures, and the application of mine counter measures (MCM). Oder also points out that the German Navy needs to get its act together and get the submarines back to sea.
Finally, Oder points out the need for regular maritime patrols in the Eastern Baltic. He mentions preemptive detection of Russian minelaying as a purpose, but I think many others could be mentioned like general surveillance, Baltic Sea Policing, show of force, and intelligence collection. Oder suggests using small ships like the Ensdorf or Frankenthal classes in order to assume a non-threatening posture. As someone who has spent my share of time on a small boat in bad weather in the Baltic Sea, I can testify that these ships are not suited for the job. You need something bigger like the German Braunschweig, the Danish Knud Rasmussen, or the new Finnish multipurpose corvettes.
Russia would want to close the Baltic Sea during the early stages of a war in the hope to discourage the West from continuing a fight. I have described this more generally in my outline of the military scenario that would unfold in case of a war between Russia and the West. Oder mentions the same point about Russia seeking a fait accompli, but he somewhat blurs the picture by also hinting at scenarios for Russia to use this ability in the absence of war, which I find most unlikely.