What does fb tauros do, I’m not really an rbyer
Fire Blast has 30% burn rate in RBY. That said, I guess that FB Tauros can do stuff like:
1) Burn the enemy Tauros on a ditto. Burning a Tauros will permanently drop its attack, since nobody uses Rest on Tauros (which is the only effective way to get rid BRN/PAR/PSN status on RBY - opponent using Haze is another way, but nobody uses Haze in RBY).
Since Tauros is the best physical sweeper of the meta, a burned Tauros is a worse Tauros. However, it does not means that BRN Tauros is useless: it can still paralyze foes with Body Slam and rely on critical hits to deal heavy damage, as CH always ignores status changes (including Burn's ATK drop).
I'm not playing, but probably this one is the greatest reason to ever consider using Fire Blast on Tauros.
2) FB Tauros can also burn another pokémon that enjoys using physical moves. Rhydon, Snorlax without Rest and Golem are pokémon that hates burns, specially because they are slow pokémon, and slow pokémon means a lower critical hit rate.
However, if you are against Golem/Rhydon, you'll prefer to use Blizzard and just kill them with it. So, you're probably not gonna burn GolDon with Fire Blast, unless the enemy switches to one of them against Tauros (which is usually a very poor move).
Burn will also bother Zapdos, as it reduces Drill Peck's damage and makes it less threatning against Exeggutor and Reflectless Chansey.
3) Burning a Restless partial-trap user is a way to deny their utility, as they will take BRN damage on every turn they attack. So, if the foe switch-ins Restless Cloyster, Victreebel or Dragonite against a Tauros, they might get burned and reduce their usefulness.
4) Burning's damage helps fellow partial-trappers to sweep. Since burn damage happens every turn, moves like Wrap and Clamp are going to be more effective (Wrap damage + BRN damage every turn). So, if you happen to burn a Starmie and is successful at using Agility on your Dragonite in another turn, you'll kill Starmie much faster than without a burn. A Dragonite can kill a burned Gengar with Wrap, despite the fact that Wrap itself won't deal any damage.
This strategy is more effective when you burn slower pokémon, such as Chansey, Rhydon/Golem, Exeggutor and Snorlax, as you don't rely on Agility to make it work. Cloyster and Victreebel does not have Agility on their movepool, so, they won't enjoy any benefit if you launch a burn on Starmie (however, that will help a lot to deplete Chansey's life).
5) Burning a ReflectLax will force it to use Rest eventually. You can use a burned or Resting Snorlax as a setup bait to AgiliWrap Dragonite (Body Slam's para is the greatest risk you'll take here if Snorlax doesn't switch on the turn he gets burned).
Or, you can use that Rest turn as a free turn to do anything you want.
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I really think that 1) is the reason why they are using FB Tauros, and maybe 4). For the last case, BRN+Wrap combo was strong enough to change Onix's status from "complete garbage" to "hey, it can do something!", using Toxic/Bind/Explosion/EQ or Rock Slide; in this case, the strategy used PSN+Wrap, but the effect is roughly the same as BRN+Wrap. Usually, Starmie is the pokémon that you DO NOT want to burn, but with Moltres, the combo Agility + Fire Spin + Burn may turn over the table and make Starmie faint against the fire bird (so, a Moltres player actually wants a burned Starmie).
However, that's a very high risk strategy, since partial-trap moves are not accurate (Wrap has the highest accuracy among them, with 85%). And if you take things like Starmie and Agility setup turns into account, the risk is even higher.
Fire Blast itself is risky if your foe has a Frozen pokémon, as a pokémon on RBY will only thaw by using Haze against it, or attacking it with a Fire-type move that has a chance of burning the foe (Fire Spin won't thaw a pokémon).
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EDIT: Most of these strategies has great flaws, and they are the reason why FB is not used on Tauros. Tauros uses its standart moves most of the time (Body Slam/Hyper Beam/Earthquake/Blizzard), as it has a great coverage against most pokémon in RBY OU. Drop one of these moves for Fire Blast is risky as well, and the reward may not be so great.