A Land Scout is the most basic unit but they are necessary for their radar range. Scouts are cheap, but extremely weak in combat and unable to stand up to even a light assault bot. During early land battles, they allow other arty and other units to shoot farther.
Offering good view coverage and reconnaissance for their price and availability, they can still serve a function even into the late game as living shields and diversions, as well as spotting for artillery, missiles, and so forth. Their rock-bottom build times and prices are second only to wall sections.
All Land Scouts are fast and have radar of a reasonable range - making them a viable means for basic base defence early on. giving you an early warning of what's to come, as well as a tiny gun which is only useful for scaring enemy scouts. Typical uses for T1 Land Scouts would be keeping them in a battlegroup to allow units to fire before the enemy comes into range, or using them as an early-warning radar system instead of producing T1/2 radar to cover a base.
When doing this, note that they are still easily destroyed, and so should only be used in the early game when producing a radar installation is often more difficult and time consuming than producing scouts. They still can be useful later; building a large mass of them (which is very, very easy) and advancing them towards the enemy can be a very effective distraction while a real assault force hits elsewhere. They can also become bait for enemy defense structures to attack first, forcing overkill and allowing your main force to hang back and bombard the enemy structures. Said structures may gain veterency but as always time is more valuable than anything in Supreme Commander.
The Cybran land scout does not have a weapon at all, instead having the ability to cloak itself, meaning that it is only detectable by radar and Omni. This makes it exceptionally vulnerable to other scouts in the early game, but until your opponent has radar set up, it can be left at mass points or other strategic locations to watch for troop movements without the enemy realising that you are watching them.
The Seraphim combat scout is really a general purpose unit. It has both cloaking and stealth, meaning that if it remains stationary and does not fire, it will be completely invisible to anyone without Omni. It can also be used for flanking, and ambushing, as its weapon does reasonable damage (although is still roughly 1/3 that of other T1 Light Assault Bots).
The UEF and Aeon land scouts have almost identical stats - the UEF one costs more and has more health, whereas the Aeon one can hover and moves slightly faster, allowing it to be positioned over water, increasing its effectiveness.
As of patch 3603, the UEF land scout has its damage doubled to 4 damage per shot, the Aeon land scout's health decreased from 23 to 20, the Cybran scout is built by default with its cloak deactivated and turning it on costs 5 energy per second, and both the Cybran and Aeon scouts have their radar range increased by 5. Although this increases the effectiveness of Cybran and Aeon scouts to do their job of collecting intelligence, this allows a UEF scout to consistently beat away both enemy Cybran and Aeon scouts without light assault bot support and survive longer to provide intelligence, even if it's range is slightly shorter.
A confusion tactic could involve a large amount of scouts surrounding a factory. If the opposing commander has radar of the area, they will possibly believe that there is an army of engineers building an army of units, while in reality, it is building scouts. Or if this fake army is moved within radar range of an outpost or base, the opponent may move their ACU away from the area, so bombers or gunships could attack it.
However it is more likely that when your opponent is aggressive, they will attack the army with one of their own that isn't made up of scouts. In that case, the scout army will be slaughtered and the enemy will gain veterancy. A counter to that scenario is by self destructing the units so they don't become free kills.