[jiggawatts]

It can be a test of the response.

If you have a plan to sever every major fibre link into and out of some country then the only way to rest the effectiveness of your plan is to sever just a couple and see what happens.

The Cold War equivalent is to fly up to the border and see how the enemy fighters intercept your bombers.

Alternately it’s “making a concrete statement” in response to a thinly veiled threat.

E.g.: if a western government threatens to cut Russia off from the Internet they can cut a fibre to two to make a point that the west is just as vulnerable.

[jemmyw]

The problem with seeing it as a test of response is that the test changes future responses. Where previously it might have been possible to do a surprise attack on cables, govts will now (hopefully) be looking at ships maneuvering near by with more suspicion.

[jiggawatts]

The problem is that in some areas there are both cables and ships in such a high concentration that it's impossible to police.

Generally undersea cables "land" at major port cities because that's where it is convenient and also that's where the data centres are.