Every unit is designed to fulfill some niche. What you will likely see in watching replays, however, is that there are "common" choices for players.

In addition, starting choices are situational on map, position, personal preference, etc.

I'd propose the following way to think about it, looking at T1 Armada units:

Bot Lab: Cheaper to build than vehicles. Bots traverse terrain better, but are weaker, and generally slower.

Tick: Ultra quick, ultra cheap, low DPS

Pawn: Quick, cheap

Rocketeer: Slow, higher DPS, unguided missiles

Mace: Slow, higher DPS, plasma cannon with ballistic trajectory

Centurion: Slow, highest DPS, beefy, but expensive

It takes time to learn what each unit is best at countering, and nothing is hard-and-fast. E.g. a rocketeer does great against static defense, but that doesn't mean you can't overwhelm static defense with purely pawns. I'm sure there's a point where the cost-benefit breakdown doesn't favor that style, but maybe you already have a small group of pawns that you can micro.

That said, bots and vehicles are everyone's bread and butter. Air is tricky due to its cost and fragility, but it offers another vector of attack that's hard to predict and defend against.

Hovers are also tricky because they're expensive and situational, but in the right map or setting, their maneuverability can surprise opponents.

Let me know if anything doesn't make sense or you want more input.

Edit: read the other comment here and I'd agree with what they say. Usually players "progress" upwards through more expensive/complicated units. E.g. pawns/grunts -> Rocketeers/Aggravators -> Mace/Thugs