If I understand it correctly, only thing stress relief does is to push fun bar up to half point. But, I think items that give fun can push fun bar all the way up, and increasing fun already reliefs the stress. Is there any reason I should invest in stress relief, or is it really as pointless as I think?
Stress Relief shouldn't be viewed as an attribute when comparing objects per se, but rather as an ancillary benefit to the primary use of the item.
What Stress Relief does is counter the "Stressed Out" moodlet, and you'll notice that it appears on beds, chairs, and bubble-baths. You'll note that none of these items restore Fun, the lack of which causes stress in the first place.
What this means in practice is that the Fun bar has two "halves". The first half, from 0 to +100, and the "stress-related" half, from -100 to 0. Stress relief will help push you closer to that midpoint, at 0 Fun, but it will never help you cross it. However, it can be combined with fun boosting activities - watching TV while on a stress-relief chair, for example, and it can counter fun-reducing tasks if done on them - writing a novel while on a stress-relief chair, such that a sim is either able to become unstressed faster, or avoid getting stressed for longer, respectively.