there is no real practical use for it, just that it would be interesting to see implemented. this makes the fake players more like real players.
suggested use: /player Steve chat Hello World, which prints "Hello World" to the chat as if the player were real and typed it themself.
maybe if they have op permissions or cheats: /player Steve chat /gamemode survival or something like that, this executes /gamemode survival /execute as Steve at @s run gamemode survival works
I'm not completely set on "chat", maybe "say" and "execute" for chat and commands, respectively

/execute as Steve at @s run say Hello isn't what you are looking for?

The /say command has square brackets around the name of the entity executing it:
[Alex] Message

An actual player chat message has the triangular ones:
<Alex> Message

Clicking on the name of the player in a message also auto fills a /msg command. You could replicate this with tellraw, as it has a command suggestion feature, and can display almost any text.

the /execute one works for commands, but i think having something inside /player for chat that displays the triangular brackets would be more intuitive I think

Maybe a command like /tellraw @a [{"text":"<"},{"selector":"####","clickEvent":{"action":"suggest_command","value":"/tell #### "}},{"text":"> Message"}] (replace #### with the player's name) would produce the results you want? But that would be complicated as you would need to write a separate such command for each player, while /player #### chat Message would be more simpler and less work on the user end.

How is this necessary. Just do
/execute as Alex run say This is how we do it -> (in the chat) [Alex] This is how we do it

Ah wsit I see what u meam with triangular brackets. I mean, idk why its necessary, but gnembon will be the judge of that