Hi Kmann, apologies if this is too simplistic an explanation but you did ask!
The anti-roll bar is simply a bar that links the 2 wheels across the same axle (so one bar at the front linking the 2 front wheels and another for the rear wheels).
Its purpose is to help keep both wheels in the same vertical alignment. I.e if you go round a bend then the van weight will load the 'outside' suspension so will compress this more than the 'inside'. Because the anit-roll bar is linked across the axle then it tries to even out this suspension travel across both sides and hence reduce the body roll.
It is simply a metal bar and other than aiding with the body roll it does NOT effect the suspension spring rate, damping or ride height. A heavy duty ARB is just a slightly thicker (so stronger) bar so will resist the deformation a little more and hence give less roll.
In my opinion it should be standard as it improves the handling/safety of the Cali with no detriment to the ride 'comfort'.
It definitely DOES make a noticeable difference, though the rear upgrade has more benefit than the front. The rear is also much less work to fit so if you were retrofitting I would start wih the rear as this gives the majority of the gain.
The T5 and T6 has the same suspension design so it can be retrofitted to either on a easy DIY basis.
For what it's worth here is a copy of a posting I did sometime ago regarding retrofitting the rear...
Regarding the uprated rear ARB i think this is well worth the money for the improvement to the body roll with no detrimental effects to the ride (will do nothing for the sag, your Pedder's springs will sort that out). The VW part no. Is 7E5511407A (was 7H5511407C) and comes with all the required bushes.
It's held on with 4 bolts (2 of them are torx). Just keep the rear axle level when fitting it, very straightforward though I can't speak for the 4wd. (From memory I'm pretty sure they go straight onto the 4motions as well). Recall fitting it without any jacking up and wheels still on, about 30mins including the first time faff. The diameter of the the arb is upped from 24mm to 28mm and is noticeably heavier. You can get it a little cheaper from TPS if you have one near you else it's your VW dealer at £112.
Attached some old/new comparative pics...
Good luck,
Cheers, Will.