Reward linking questions to leverage and organize existing knowledge
There is already so much knowledge stored inside the Stack Exchange network. The chance is high that an answered question relates to a new one. And it will only increase:
The longer a knowledge domain is covered by SE, the more basic and simple questions will have been answered. Automatically new questions tend to become more niche and sophisticated over time. But they don't pop out of thin air; they build upon established first principles and prior art. And when answered turn to part of that themselves.
In summary, the present knowledge will become always more important in relation to new questions and of course in general through its sheer extent. To benefit users the most, it should be accessible in the easiest way possible. The web came up with hyperlinks for that, which have a special instance on SE as linked questions.
To make the collective knowledge of SE more accessible, linked questions are the web's native, low barrier way. They create the eponymous navigatable web of wisdom and give context without search. There should be an incentive to create more of them to emphasize the wiki like character of the network.
How is this different from Reward finding duplicate questions?
This is another topic. Duplicate questions are alternative phrasings of an existing question. Linked questions are other questions related to an existing question.
The mechanisms, consequences and dynamics behind are completely different:
|Aspect||Linked question||Duplicate question|
|Trigger||Answer/comment with current site link||Flag existing question|
|Appearance||List item on the right||Top center redirect|
|Consequence||More context available, more interconnection||Question closed|
|Incentive dynamic||Depends on implementation||Built in Fastest Gun in the West|