I may also be missing some nuance depending on the build, but the main thing is the extra HP. Things can damage you without you being in melee (enemy ranged attacks, casters casting spells, traps, environmental effects, etc). If it means the difference between getting one shot, and surviving one hit, even just barely, then it's worth it (cause at least then you can continue fighting and have a chance to recover without waiting on death timers)
At level 34, every +2 to Con is an extra 34 HP.
At level 34, each Toughness feat is +36 HP
Epic toughness is +50, but you must be level 21 and have a con of 21+ and have already taken normal toughness as well
Legendary Toughness is +100, but you must be level 31 and have a con of 21+
Heroic Durability bonuses from combat feats vary by class as they scale off your base hit dice. They also scale as you level.
For Alchemists:
At level 34, having one combat style feat nets you 47.5 extra HP
At level 34, having two combat style feats nets you 95 extra HP
At level 34, having three combat style feats nets you 142.5 extra HP
At level 34, having four combat style feats nets you 190 extra HP
Now, why choose SWF over Shield master or TWF or THF or Natural Fighting? Prerequisites.
Natural Fighting requires druid or blightcaster levels, so that's out.
All the others have BAB requirements. TWF, THF, and SWF go up to +11 BAB, while the Shield line goes up to +13 BAB, though the first in each line starts at +0 BAB required.
The Shield line also requires proficiency in shields first, which may cost an extra feat as well.
TWF line eventually also requires a Dex score of 17+, but starts at 15 required
THF line eventually also requires a Str score of 17+, but starts at 15 required
SWF line eventually also requires a Balance skill score of 7+, but starts at 2 required
The Perfect feat for each line only has a level requirement of 22+ and having one of the combat style feats in the related line.
Balance is a class skill for Alchemists, so it's really easy to get, and well worth the 2 skill points, especially compared to trying to spread out ability points to stats you don't use (str of dex) for the other lines. Ability points are much better spent on main stat and con, then and only then putting points into others with what is left.
So (ignoring legendary toughness as that can be taken as an extra regardless), one could take toughness and epic toughness for a total of +86 HP at level 34.
OR one could take SWF and PSWF to gain +95 HP at level 34.
I don't recall off the top of my head how they all interact with % based boosts though, but from a plain comparison, the combat style feats have the clear advantage.
Here is the
wiki entry for Heroic Durability, which is what the combat style feats modify, and it includes a table that shows the values of HP gained by each class for how many taken and at base, level 20 and level 34.
