On What Occassions Does One Use Conduit For Cabling?

We use Conduit whenever we run cable outdoors and/or where we are concerned about vandalism.

When installed correctly, Rigid conduit does not entrain water. It's just like pipe; same unions, same thread, etc often can be used. Rigid is used to run cable into hazardous environments because of the performance in sealing off cable from the outside hazards.

Shielding is another issue. Typically shielding or STP is used in locations where outside EMI is a concern, like adjacent to high-voltage AC machinery or to protect against lightning interference. See: STP vs UTP for Surveillance for more.

I believe Axis cable spec for their outdoor PTZ used to recommend shielded cable. Have not looked at current spec in a while.

...it was suggested that is better to lay cable without conduit as water collects and causes the cable to deteriorate.

You can also use direct burial cable. Or DBC in conduit if you want to take no chances :)

Related: Conduit for Surveillance Explained, Cable Trenching for Surveillance

If water ingress is a real concern (such as in weather-exposed locations, particularly running across rooftops and the like), we'll use PVC, and junction boxes specifically designated for wet locations.

Outside, EMT is fine if it's not going to be in standing water or something, and the main purpose is protection from physical access.

Inside, EMT is definitely the spec for new installs - just so much easier to work with in most cases, vs free-airing bundles of wire over ceiling structures and HVAC and the like. We did a couple of jobs free-air recently because the client didn't want to pay for the piping... both were nightmares, especially on one where later construction damaged several cable runs, requiring a couple extra days of labour to repair or work around.

My personal thoughts:

• Construction/remodel:
• In drop ceilings inside secure areas (i.e. inside the card access perimeter)? Never. Waste of money. Utilize cable trays and run hook lines along pre-determined (via plans) areas, and clearly mark your cable so it doesn't get demo'd or damaged.
• In drop ceilings outside secure areas? Always. How can you be secure if someone can clip a wire in a ceiling?
• Inside walls to device locations / backboxes for card readers / pipes+ring and string to hinges/headers, etc? Always. Massive time-saver and protects against the hazards of construction.
• Retrofit (not during construction)
• In drop ceilings? See above on both counts.
• Inside walls? Never. Cost is entirely too high. Fish walls and doors.
• Outdoor, buried:
• ALWAYS pipe + direct burial cable. Not one, not the other -- both. Cost be damned, because it will cost 20x that much later.
• Outdoor, exposed to water:
• Pipe only (no direct burial), anywhere that water can feasibly access.
• Outdoor, not exposed to water:
• No pipe or direct burial. Free air.