Want curvy maps but hate wandering vertices and compile hell due to wacky geometry? Displacements can replicate this using very little.
Ok, so say you wanted to turn a boring normal corner like so:
Into a rounded area to make it more interesting. Usually you could use the arch tool or use brush rotation as per jurf ramp creation to create a smooth rounded surface. However, the more smooth you make the curve the higher brush count required. Additionally the geometry will usually end up being a func_detail or _brush to avoid expensive vis calculations.
Instead we can turn the visible faces into displacements.
Then if we hit the magical subdivide button we end up with a curved corner!
But! The top and bottom of the wall has awful curves which are a result of the way subdivision works. However, you can 'fool' hammer into not doing this by making 'templates' of the brush faces you are subdividing. Simply copy/paste the same faces you are subdiving above and below the original faces:
Now hit subdivide and you will end up with a much smoother curve, and crucially, the ugly distortions are still only at the extreme top and bottom of the new 'template' brushes:
You can then simply delete the template brushes leaving you with a smooth curve
Critically this only takes 2 displacement faces to replicate what might take upwards of 16 brushes. This can also be used to create curved pathways and much more. Additionally, its very trivial to make smooth cylinders using subdivision without even the need for templating, such that the cylinder tool is pretty much worthless unless you need to have an opening in one.
The main downside is that displacements are not the best for collision physics so if you require a super smooth wall which the player will move against the surf ramp method or careful use of the arch tool may be better.