Noob question: what is the actual use of regular infantry?

I'm just starting to play the game seriously, but I struggle to find a use to the regular infantry unity (Militia and its evolution).

• Early game rush usually have archers, since they can collectively can "one hit" other units, if you have enough of them. Also, they have advantage against Infantry.

• Castle age then people go for horses, since they are fast, and good against archers.

• If I want to go full defensive, I would focus or garrison archers, or spearman/pikeman to counter whatever the opponent might do.

• Infantry is also weak against siege.

I know that they are the good "king of all trades" stuff, but I fail to see whenever I should focus on infantry.