I don't know if anyone else uses it this way, but I use Sym turrets as a form of 'slow', either on the choke, a flank, or to form a protection holding zone for the supports. At the choke it's good for farming some ult, annoys the tanks since they have to waste that 1s to clear the turrets which your team have a free 1s to shoot at them every engagement. It slows down progress a little, not a lot, but sufficient. Watching a flank with 1-3 sentries is typically good.
For the localized protection, it's pretty useful to hide obscure turrets near your supports default holding location. It deters a lot of flankers if you don't clump them up and make them have to 3 shot your turrets. The slow effect also buys time for your healer to run. I've had plenty of times slowing down Genji / Tracer enough to right click them (yes right click), then M1 them to death because they were unaware in their first engagement. Every other time they try to engage, they'll have to deal with the turrets which in turn means they can't chase the healer quick enough. Also lol at Genjis trying to Dragonblade in only to get stalled by the turrets from different directions.
The 'slow' aspect of the turrets are really underrated though. It's as close as it gets to rooting people in an area. Oh and Winston can't deal with turrets that are spread out further than his tesla cannon range, he'll get slowed too much. Some maps allow for really long distance placements that allow the turrets to still lock on and yet not get wiped out instantly. But if that terrain doesn't allow effective placement for that, yea Winston will still wipe it, or any explosive basically.
General rule of thumb is to build far apart that one Reinhardt swing can't kill them all, one Zarya right click, and roughly tesla cannon range. And split your turrets to do different jobs.