All forcefields do is make an area on the ground unpathable and push units already there away. But forcefields have provided for some very emergent gameplay. They can be used for anything, in battles, cut retreat paths, sneaky ramp block warp prism tactics, luring someone in to their main with blink stalkers, thne blinking out and forcefielding the ramp, keeping dts out of your base while you tech to observer. Save a red nexus surrounded by lings by spamming forcefields around it. They can even be used to cancel nukes by pushing the ghost away you can't see.
Forcefields are pretty much the only ability that actually takes skill
We're all cynical about 'great emp', 'great storms', 'great fungals' while there's nothing that amazing about it. But actually getting a line of forcefields down in a half a second with zero overlap in the right position, that's actually pretty darn hard. Forcefields are an ability with a lot of degrees of success from a lot of overlap and gaps to no overlap at all and in the perfect position.
Forcefields do not do damage
Despite its enormous potential, they do not do damage, this means that forcefields do not stack and sentries are always going to need support, no matter how good forcefields are in the hands of a master, pure sentry, unlike pure infestor, just will never fly.
Forcefields bring finesse
Rather than just applying the sledgehammer, massing roaches and going for it. A forcefield-supported army has more finesse to it, the units are weak on their own but a zoning and divide and conquer strategy is used to in the end come out on top. Engagements are no longer about brute force but about very surgical partitioning and positioning.
Forcefields force engagements and micro to matter
Let's face it, without forcefields flanks would be bloody irrelevant against the protoss army. You'd just pushg your roaches up against it and they'd all fire anyway. Forcefields make flanks and positioning matter. Make terrain matter, make zoning out the army of the opponent to deny a good position or acquire a good position to fight in relevant. Every Zerg knows the horror of Protoss getting into that position at the third on Bel'Shir vestige. The solution: Don't let them get there, engage on favourable ground before they get there, it means you have to be mindful of the location of your army because forcefields make positioning and terran matter that much more.
Without forcefields, ZvP would simply be roach vs roach, micro and positioning would hardly matter any more, it would just be stalkers and immortals amoving into roaches and hydralisks for the most part.
Forcefields make engagements last longer
Forcefields reduce the collective dps of the enemy army, of course they make engagements take longer. In your typical PvZ 2base all in when it starts no one knows who's going to win, they can take super long, some up to 7 minutes and it's generally close until the end. It doesn't just come down to composition and numbers, it comes down to micro and positioning from both sides.
Forcefields create hype
How many times has the crowd gone wild over a 4 hp colossus surviving because forcefields blocked the marauders of from killing it? From MC just walking a sentry across the map to randomly forcefield a ramp and killa nexus with 2 zealots and a sentry? From 20 roaches being trapped inside forcefields gone down in a second by a master? As much as people say they hate them, they go wild when they are used in an oppressive manner.