I've done a couple sieges including in several opendev scenarios (Norse and English). If proper units are garrisoned behind walls or fortifications, dislodging them can be very troublesome without proper siege units. Fortification alone provided +6 str. to the defending units that drastically reduced overall damage without factoring in friendly adjacencies.

The AI didn't handle sieges well in Lucy as these types of combat are inherently different from field battles. 

- AIs didn't keep units garrisoned in cities to ward off aggressors. 

The fortification bonus is massive and enables standard units to defend vs. stronger units. Taking such fights without taking down fortifications can be very costly.

- AIs moved units "normally" and doesn't keep units behind fortifications. 

This goes for both attacking a sieged city or defending a city from a siege. After units are deployed, the AI will typically push out of their city to engage (getting slaughtered) instead of holding up. When a player attacks a sieging army from a city (sortie), the AI will just keep plowing units into the fortified units and suffer greatly. 

If an AIs city is being attacked, it should keep its units behind walls while the attacking army slams their head against the walls. Sure, some powerful and ranged units can engage the siege, but generally it needs to muster troops or get reinforcements before attempting to break a siege.  

If the AI is engaged by the defender in a sortie, it should wait until the units come-up into the open field. The AI has little reason to be the aggressor here especially because the cities defenders will automatically retreat out of the city if they do not win (or capture the flag). 

Siege units themselves (Ancient: Ram 29 str., Classical: Ballista 38 str., and Medieval: Trebuchet 41 str.) all have superior combat strength for their respective eras. Multiple units will be necessary to stop them from quickly knocking down walls and fortifications. Once enough walls are down, the ranged siege weapons can be used to heavily injure the defenders units.

I believe it is wrong to say siege doesn't work as intended or have a place in Humankind. The backend systems are still a work-in-progress (AI included) and such systems will be untilized different vs. other human players. 

At least it was very useful when the enemy have (in Early Modern Era) built every single defense infrastructure within the City. With all the infrastructures, the Fortification was far more stronger than the Combat Strength of Mortar Units. Thus, decreasing as many  garrison Units as possible before the Battle was a viable strategy.