[thyristan]

I can see a few advantages:

Internal Propaganda. You show to your own people, that you can cut off the enemies' communication lines easily.

External Propaganda. You show to the enemies that they are vulnerable, spreading fear and doubt in their own strength.

Exercises for larger operations. You train ships' crews for those kinds of maneuvers, in case you need to to it a large number of times, e.g. to cut off all baltic cables at once, cut all transatlantic cables, cut all cables to some important island like Iceland, etc.

Internal Normalisation. You get the ships' crews, your population and your governance structures used to a more aggressive mode of operations.

External Normalisation. You get the enemies' population and governance structures used to those kinds of pinpricks. So when the large-scale operation starts, they will ignore the first signs as "just the usual irrelevant pinpricks".

Testing and mapping connectivity. When the cable goes down, you can have your spies look at which relevant infrastructure goes down at the same time.

[The_Double]

This is one of the best, succinct lists of motivations for hybrid warfare i've seen.

But i could suggest another potential benefit for russia: If russia already operates under the assumption that they are in a (cold) war with EU/NATO, and they don't care about the effects on the relationship with Finland. Then this may simply be a really low cost, high damage operation. That not only imposes the replacement cost of the cable, but also forces countries to invest in counter measures.