Why does Double Move have to be the same move action?

Question

I only just noticed this particular rule in PHB p. 284:

Same Move Action: To double move, you have to take the same move action twice in a row on the same turn.

What is the reason for this specification?

How (and how much) does it affect/break the game if a player uses different move actions for Double Move?

Answer 1

You can still spend your Standard Action on a second Move Action without using the Double Move. ("Substituting Actions", Player's Handbook p. 268).

The Double Move is a specific action where you effectively combine two identical Move Actions to one movement. This is only beneficial in a few cases, usually the following:

• When you use Athletics to jump, and the first movement would leave you in mid-air, you can spend your Standard Action to make it a Double Move to complete the jump in one fluid motion.
• When your movement speed is an odd number and it is halved by difficult terrain, you waste the last square of movement because halves are round down. Double Move lets you add together the two halves and squeeze out an extra square of movement.
• If you want to move twice but can't because the first movement would end on the same square as an ally, a Double Move lets you do it because you don't stop in the ally's square.