The Assault is a versatile, cheap and very fast unit mostly used to collect containers on the map.
Very cheap
Strong vs most infantry
Effective vs lightly armored vehicles
Very fast
Very accessible
Weak to any kind of aviation
Slower than Rifleman
Weak against heavily armored vehicles
For Confederation players, using assaults to collect small containers at the beginning of a game is the only option, as the only other unit which can be built during deployment time ( Fortress ) is very slow.
As long as the enemy uses only a small number of Coyotes to fight for the big container it is possible to stay with assaults only, as long as they outnumber them 5 or 6 to 1 (always use 3 Barracks on land maps).
In comparison to the Resistance's Rifleman, an Assault is slower, cannot move through forests and costs more, but has a noticeably higher combat power in turn. This means that it is always a viable option to counter infantry rushes with assaults as long as the enemy doesn't use Snipers to attack you. If this happens you need to use Fortresses, Cyclones or Fire Assaults instead.
As mentioned before, Assaults can't hide in forests, but that doesn't mean that you can't scout with them. First off, they can be hidden in the corner behind the enemy base on a lot of maps to later on scout them after they already secured the front side of the base with Sniper Towers. The information you get that way is invaluable, especially if you consider that you don't need to produce a more expensive scout unit instead. Another way, mostly used by higher ranked players is to spread single Assault units all over the map to spot enemy attacks early.
Assaults are great at supporting fast anti-vehicle units like Hammers, as they can not only keep up with their high speed but also provide much-needed defense against Grenadiers / Heavy Assaults.
They are also able to be run into Chameleon clouds that are stationary due to their high speed. This allows them to reveal the units inside temporarily while also drawing some of their fire on themselves.