One of the main features of 0ad that serves to make civs often play very similarly is that many units are totally equal for some civs. Another issue is how simple many unit roles are (think meatshield/sniper meta). Unit specific upgrades is an attempt to differentiate unit roles and create some situational upgrade clicking as opposed to "must get" like the current military upgrades are. Unit specific upgrades could also help create more specific roles for units, depending on the situation.
Considering civ balance and civ playstyles, some civs might not get all the unit specific upgrades for each of their units. This would make them maybe not "unique techs", but certainly "non-universal" techs.
Consider that in Aoe2 there are many similar units (people who like games to be "cool" don't like this, hence aoe4). Japanese get every tech that is available for cav archers, meanwhile a couple other civs have access to less techs but have specific bonuses regarding cav archers. This creates differentiation in when and how cav archers are best used for that civilization. I'm not saying that we should be Aoe2, but I am saying that this is a very effective way to make "the same" units play very differently.
To give an example from earlier, someone mentioned "longer pikes" (one of the unit specific upgrades). It seems like it won't have much gameplay effect, but the crucial aspect is that is has greater effect in certain situations (ie chokepoints, getting hits on cav). This is really fun compared to the plain old +armor, +damage techs.