I know Mika Hakkinen left a race seat for a test role at McLaren after (I think) two seasons. And, like Alonso, it didn’t do him much harm, did it?!
Presumably you mention this because of Nico Hulkenberg’s situation. I think he should consider moving to Mercedes to be their 3rd driver. OK, so he’ll get next to no test time, but I think it’s safe to assumer Michael Schumacher is closer to the end of his career than the start, and therefore a vacancy that looks like it was made for the Hulk will be coming up in the near future
Would it be because the top teams spot talent and recruit them to their team, spend a year or so developing their setup skills and technique as well as forming a good working relationship with engineers. Also, because they are in a top team, they often have a good car underneath them when they get a race seat in that team.
Although I wouldn’t claim he was the best, and his situation was also rather different to those already mentioned, Timo Glock is worth a mention – after making a few stand-in performances for Jordan in 2004, replacing Pantano, he spent a year in Champ Car, two in GP2 of which he won the second, and returned to F1 arguably a better and more mature driver than if he had simply stayed after his debut…