Most shared hobbies develop by discovery: you find out that a friend also runs, also cooks, also plays chess, and you begin doing the thing together because you were both already doing it. This is fine. What is less common, and more interesting, is the hobby you share on purpose — the activity you adopt or introduce or sustain specifically because it gives the friendship a recurring occasion that would not otherwise exist.
The distinction matters because the discovered shared hobby depends on pre-existing alignment that may or may not exist with the friends you most want to maintain contact with. The deliberately shared hobby is a choice: you decide that this person is worth the investment of a new activity, or the continuation of an existing one in a new configuration, and you build the contact structure accordingly. You take up running because your friend runs and you want a reason to see her every weekend. You propose a monthly cooking night because you and your closest friend from work have lost the proximity that sustained the friendship, and you need a replacement. You start playing tennis again, after a decade away from it, because the friend you want to see is a tennis player.
This is not instrumental in a cynical sense. It is strategic in the way that any adult maintenance of important relationships is strategic: you recognize that the friendship matters, that the conditions that previously sustained it have changed, and that if you don't create new conditions it will thin. The deliberately shared hobby is one of the tools available for creating new conditions.
What a shared hobby provides that pure social contact does not is what researchers call parallel activity: doing something alongside another person rather than simply being with them. Parallel activity reduces the social performance pressure of unstructured togetherness — the awkwardness of a conversation that has to be manufactured from scratch — by giving both people something to be doing, which allows conversation to arise naturally from the activity rather than being the sole purpose of the encounter. This is why fishing trips produce more honest conversation than dinner parties, why people tell therapists things walking that they won't say seated across a desk, why the game of chess or the kitchen or the trail has been, throughout human history, a vehicle for the kind of talk that needs something to happen alongside it.
The deliberately shared hobby also creates a recurring occasion with a built-in reason to meet that requires no new social initiative after the initial commitment. Once you have established a Sunday run with someone, the Sunday run is the commitment; neither party has to generate fresh initiative each week. This matters because, for most adults, the social initiative problem is real: even for friendships that both parties genuinely value, the activation cost of reaching out and proposing contact is a non-trivial friction that can prevent contact from happening. Hobbies eliminate that friction by providing a pre-committed recurring structure. You don't have to decide to reach out; you just have to show up on Sunday.
The question of which hobby serves this function depends on the friends and the friendship. The relevant criteria: a rhythm that matches the desired contact frequency (a weekly run sustains different contact than a monthly cooking project), a format that suits the people involved (introverts generally do better with activities that don't require continuous direct conversation; extroverts tend toward activities that generate talk), and an activity that both parties find genuinely engaging rather than merely tolerated for the sake of the contact. The last point is not trivial. A shared hobby that one person endures for the sake of the friend is likely to be abandoned; the hobby that both people actually want to do will sustain itself even through scheduling difficulty and competing demands.
The deliberately shared hobby is not a hack. It is a recognition that the conditions for friendship maintenance do not arise automatically in full adult life and that creating those conditions is the work of the person who understands what friendship requires.