Yeah, I know. It doesn't make a lot of sense to me, either.
Now, a disclaimer; I'm not an expert with Pathfinders, myself. I'm still trying to figure out exactly what I want out of them, and sometimes they live most of the battle and sometimes they get smoked turn 2. But I've been studying a fair number of Tau battle reports of late, gearing up for a tournament on Saturday, and one of the weirdest things I see is that, time and again, cadre commanders will put their Pathfinder squad up near the middle of the board, usually in area terrain but sometimes just half-hidden behind a wall or the like. And unsurprisingly, since the utility of markerlights is not exactly a well-kept secret, those Pathfinders usually end up a cyan smear on the battlefield not long after the game starts.
It's strange, because it's so unnecessary. Not only does the markerlight possess a 36" range, and can't be blocked by cover, but it's part of a Tau Empire cadre. Which is to say, every other army out there is going to come to them. Well, maybe certain Guard builds won't bother, but pretty much everyone else is going to be angling to get close enough to rapid fire those pricey elite XV8s or blast the fragile scoring Fire Warriors off the table, if they're not aiming to get clear into close combat to begin with. There's no need, none at all, for Pathfinders to be deployed forward of the main gunline or fire base or whatever deployment a cadre has opted for, because with that range, and those enemies, even the most conservatively-deployed squad is going to be absolutely spoilt for choice.
The best way to field Pathfinders, so far as I've seen so far, is to keep them nice and back. Use them as bubble wrap for an XV88 squad, or have them hang out in the backfield with the Deathrains and the Hammerheads. Even a few markerlight tokens will lift those units from 'reliable' to 'auto-kill', and if something drops into the backfield that particularly needs to be killed, the Pathfinders are going to be right there to light it up for maximum damage.
The Tau's greatest strength is their range. Well, second-greatest, maybe; I think 'railguns' might be their greatest strength, actually. Either way, the worst thing a cadre commander can do is push forward aggressively with supporting units. A suicide melta XV8 or a squad of Gun Drones harassing and pinning is one thing, but Pathfinders are move-or-fire troops who get more valuable as the game goes on, where they can use BS upgrades to balance out the loss of shot volume. Keep them back, keep them safe, and keep them firing.