In many styles of play with Age Of Sigmar you will not have an army list that you would with most other traditional tabletop war games, instead you will likely have either a box of models ready to deploy anything you like or a pool based list where you will deploy only a portion of your selected army. In this blog post I'll mostly be considering the latter however many points still apply to the more open gaming styles.
In a pool based list such as SCGT you will have a 150 point list and deploy up to 100, with some available for summoning. It is important to have available multiple types of unit which can be deployed to best react to the current situation. For example, if your opponent deploys the Sons of Behemat Formation with five Giants, you will likely want to deploy something specific to oppose this huge threat.
Essential tools for your army list:
Objective claimer. Fairly self explanatory tool which can encompass many different types of unit. It's likely you will have some idea of the scenario you are about to play, or may even know it well if you're at a tournament and had a chance to have played it beforehand.
So you must include units in your list which can complete the objective required. For example, if you need a unit with the Monster keyword to capture an objective you must have one in your list if you want to win!
Other examples are having a tough unit/s if you're required to defend an objective rather than glass cannons. Or having a unit with the Priest keyword to make the most of a special scenario rule.
Teleport/fast unit. In Age of Sigmar movement is imperative, having a unit which can immediately react to a situation in game can be the difference between a win and a loss. Make sure to have a unit that is either flying and/or fast, or a unit that has a special deployment such as the Stormcast deploying from the Celestial Realm. This really can be important mid battle if the battle switches sides and your units can't reach an objective in time.
Hard counter. Sometimes the only option to a situation is to put down your biggest and toughest unit, something like Gordrakk, Archaon or The Glottkin. This can be a psychological counter as well as a hard counter and can influence your opponent to deploy something he may otherwise wouldn't.
Soft counter. Polar opposite to the Hard Counter you may be playing in a much friendlier game, against a new player or at Warhammer World where best opponent votes really matter. In this case it is worth having a unit or two you can deploy that will mute the power of your army a bit, something subpar but characterful that will ensure a good game for both players.
Ranged. Simple tool to add but many might forget when theming their list. A ranged unit can be prefect to take out a fast avoidance unit or a powerful unit your don't want to charge you.
Charger. Either a first turn charge or a reliable long charge can be a great defensive tool to block units which have abilities that only work on the charge like Godrakk or anything with a lance. This also works great against monsters as they worsen with the more wounds taken.
Modifier. A unit which can affect another unit with -1 to hit/wound/save. This tool is specific to oppose certain units which have abilities that only work on the roll of a 6 in game such as the Executioners, Paladins, Trolls etc. The -1 will nullify that ability completely.
This can also be used defensively with a positive modifier to protect against your opponent using the above or to enhance an already powerful unit.
In summary, there are many types of unit you will want to make available to you I deployment, more than you will likely be able to take. Therefor it's important to prioritise what units are important for the game or tournament yo are attending.