When They're Worth the Cost
For characters like animal instinct barbarians or monks with better unarmed attacks, there may not be much use in taking these abilities. But for various characters like the goblin wizard or even just a regular fighter looking for a guaranteed backup option, the benefits may outweigh the typically low costs.
Benefits of Ancestral Unarmed Attack
You've mentioned quite a few reasons it might be worthwhile to get an unarmed attack beyond the standard 'fist', including as a backup if weapons are taken away and freeing up whatever hands a weapon would use.
Having an additional free hand opens up quite a few Athletics actions like Trip and Grapple that render foes flat-footed to your entire party, as well as allowing the Grab an Edge reaction without needing to let go of a precious weapon if someone knocks you off a cliff.
Additionally as ESCE mentioned below, there's value in having multiple damage types available in case of enemies with resistances, immunities, weaknesses, or other circumstances. Dealing 1d6+10 on a bite against piercing weakness would potentially be better than 2d8 slashing with a striking weapon, even factoring the +1 accuracy.
And while a spiked gauntlet would be pretty good as an alternative for one of these cases, it deals less damage than many of these ancestral unarmed attacks and isn't finesse for characters that lack Strength.
Additionally, not all ancestral unarmed attacks deal physical damage—there's also options like an automaton's energy beam dealing fire.
Virtually all characters are at least trained in unarmed attacks, while many are not trained in martial weapons like shortswords or warhammers. A wizard with a surprising d6 bite attack might come in handy compared to drawing out a dagger (requiring an open hand) and dealing less damage. Or an alchemist firing a laser out of their eye might make a good ranged option when there aren't any bombs left.
Not to mention that with some options there are more ancestry feats associated with the unarmed attack. Like the goblin's bite in the reaction Ankle Bite and Hungry Goblin, allowing more specialization and benefits using these unarmed attacks as you level up. Whether any ancestral unarmed attack has similar synergistic options will depend on the specific case, a goblin building around that bite attack by taking those ancestry feats seems like an interesting character with advantages over other attack options.
And with versatile heritages like dhampirs, beastkin, and tieflings, these options are available to virtually any character regardless of ancestry, with varying levels of additional support through higher level ancestry feats.
There's also the simple benefit to being something like a sabre-tooth catfolk or an automaton shooting lasers out of their eye. Having an ancestral unarmed attack might be a core piece of character identity that a player will try to optimize around, rather than considering it as part of optimization.
Costs of Ancestral Unarmed Attack
In most cases, a 1st-level ancestry feat or heritage. Some ancestries like humans have great 1st-level ancestry feats (Natural Ambition comes to mind), but others have more niche uses/fewer options that appeal to every sort of character.
For one of your examples, a Catfolk adventurer may not be interested in being a bit luckier on Reflex saves, dancing, knowing the ancestral lore of their people, or fighting with their ancestral weapons. In this case, the opportunity cost of one 1st-level ancestry feat would be low, and the benefits are more likely to outweigh it.
And at the low levels these abilities are available, upgrading the unarmed attack isn't required. Retraining out of these feats when they become less useful relative to your magical weapon(s) is an option as that becomes a problem.
There's also the 3rd-level general feat Ancestral Paragon that allows characters to gain an additional 1st-level ancestry feat, granting even more opportunities to take these abilities in situations where options are limited.
Are They Worth It?
Probably not for many characters, particularly if an alternate option is more attractive, they have stronger attacks from a class, or they don't worry about the problems an ancestral unarmed attack would cover (knowing your GM isn't 'sadistic' certainly helps).
That said, characters with weaker alternative options from their ancestry/heritage/class, or who are looking for a particular attack like a laser eye could find these options good enough to take. And characters specializing in these attacks with synergistic, higher-level ancestry feats might be worthwhile on their own merit.