A Land Scout is the absolute most basic reconaissance unit you can build. They are extremely weak, unable to stand up to even a light assault bot, but they do have good radar and view coverage for their price and availability. They 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, allowing you to wage rather intense psychological warfare on your opponent.
All Land Scouts are fast, and all but the Cybran are armed with a small gun (all mostly ineffective when compared to main combat units of T1). All land scouts 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. 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, but does have the ability to cloak itself - meaning that it will only be detected by radar. This makes it exceptionally vulnerable to other scouts in the early game, but until your opponent has radar set up, it can be left in the middle of 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 etc) 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 good trick is to make alot of aeon scouts, then make them surround a factory. If your opponent has radar of the area, they will believe you have an army of engineers building an army of units, while you will just build scouts. Move the army within radar range of an outpost or base, than let your opponent move their ACU away from the spot. Use bombers or gunships to attack it as it flees.
However, if your opponent is aggressive, they may attack the army with one of their own that isn't made up of scouts. In that case, your army will be slaughtered and the enemy will gain veteran xp. counter that scenario by self destructing the units so they don't become punching bags.