I was wondering about this particular game mechanic in tactical combat games, like XCOM2, where you have two actions, which may be:
...but not attack+move, because when attacking, it ends your turn.
If I compare this to tabletop D&D 5e, with its move+attack actions, it shouldn't be that different, but this restriction seems to make things completely different.
What is the game design purpose for this kind of restriction?
One standard rule not just of game design but of any form of entertainment is to aim for an engagement curve which slowly builds up excitement and then relieves the tension at the peak point.
Game designers usually aim for that curve not just in their overall narrative, but also in smaller events, down to the most basic game loops.
When you execute the turn of a unit in a tactical combat game, then the attack is arguably the most exciting part of it. So it makes sense to put this event at the end of this interaction.
Extra Credits has a nice video about the topic of macro-pacing and micro-pacing: Pacing - How Games Keep Things Exciting.
If the player would attack and then move, they would be encouraged to move their units into a safe position after every attack before the enemy can strike back. This would encourage a very defensive gameplay style where the player takes very little risk.
On the other hand, having to move and then attack encourages a more aggressive playstyle. The player moves their units to the position from which they can attack most efficiently and hope they get rid of all threats before the enemy gets their turn.
There are several reasons why you want a more aggressive playstyle.
Allowing a move after an attack lets you aggro an enemy then dash out of its range / behind full cover where your unit is no longer at risk.
It's a safe but dull strategy that avoids the drama of injuring or losing loved/useful/heavily-invested soldiers that's at the heart of a game like X-COM.
By requiring that the soldier stay in the place from which they shot, the game forces the player to weigh the pros of a good firing angle over the cons of an exposed position more heavily, and find ways that they can use their squad together as a team to support those on the front lines.
Together with things like "doomsday clock" mechanics, this is one way that the game's design tries to constrain the player's strategy toward the high-drama intended experience of constantly pushing forward, rather than a tedious/methodical/safe approach that never costs a soldier.
But not every tactical combat game works this way. For example, in Mario and Rabbids Kingdom Battle, you can choose to spend your movement and attacks in any order. This helps differentiate the experience of this game, which is not about the drama of risking a soldier in the field, but about accessible puzzle-solving. So allowing the moves in any order both widens the puzzle-solving possibility space, and makes the game more welcoming to players by having one less restriction to teach & remember when planning moves.
