As a non-triathlete who dabbles in running and cycling, I can say that there are general benefits but the specific adaptations are different, running and cycling. In my experience, adding the runs does not help my endurance on the bike, because the time and energy spent running even just 15-20 miles per week always comes at the cost of long bike rides. So I kind of think it hurts my cycling endurance. Serious triathletes do both, so they probably don't see that cost.
Less than 15 miles per week, I'm losing ground. That's partly due to my more advanced age - we lose conditioning more quickly - but in general if you skip 2 days or more of the runs, you start to lose the gains. So, running only 2 days per week is basically just holding steady, not advancing.