Because for linked devices to work it's like a group chat. Each app must encode their messages in the keys for all devices. So if you have two linked devices and Alice wants to send you a message A she has to encode it encode(bob1, A), encode(bob2,A)

So the clients MUST know about the number of linked device keys for the protocol to work.

If I had to guess, seeing that number change might be an indication that that contacts account might be compromised. So if you see your contacts number of linked devices increase you could ask them about it through another channel/irl and confirm if it was them who linked it.

It's Linked devices to your account. If they ate inactive after a time, they are deleted