That is a loaded question.
I dead head roses to get another load but some roses have brilliant hips (not all)
I also dead head daffodils as it conserves the bulb
Clematis I forget to.
But really some plants produce great seed pods that last for ages. Others dry & change colour

As above, some plants are grown for the seeds.....anything with berries or fruit for example, honesty and teasel are a couple of good examples where the seed heads are very decorative and on some plants, you want the plant to seed for the following year.

I deadhead only the very few plants that are likely to cause a nuisance after they have seeded all over the garden. The rest stay for seed collection or to enjoy the fruiting heads. I would say that there is actually little benefit to any plant to deadhead it.

Hmm, that isn't always true, Bulba - sweet peas, for instance, will stop flowering if you don't deadhead, so its mainly the annual, summer stuff where you might want to be rigorous about deadheading, though it's not necessary for all of them (I never deadhead surfinia petunias, for instance, and they carry on flowering regardless).

I deadhead a lot of my summer flowers not only to stop some seeding and to get more blooms but also to tidy them up a lot of dead heads can spoil the look of some plants.