Basically, it's Pounce for free.
One of the biggest problems that melee classes face is the fact that, in order to maximize their DPS, they need to spend a full-round action attacking. If you spend a full-round action attacking, that means that you're moving, at most, 5 feet. In addition, it means that if you start your round far away from an enemy, you're only going to get a single standard action attack off.
There are a number of ways around this issue. There are class features that allow you to make a full attack after a charge, or teleport as a swift action, or just make your standard action attack better than it would normally be. Mythic haste gets around all that. With Mythic haste, you can move and make a full attack in the same round trivially, without expending additional character build resources.
The example that you give (move, standard action attack, move again) isn't the case that people are talking about when they say that Mythic haste is awesome. More often, they're talking about using a move action to get close and then full attacking. Alternately, using a full attack, killing an enemy, and then moving to close with a second enemy in the same round.
Abilities like this usually take up significant character resources to acquire. With Mythic haste, all you need is a spellcaster with access to haste who is willing to use one of their Mythic feat slots on Mythic spells.
This interpretation assumes that the move action granted by Mythic haste is in addition to your normal full-round action. Any source that describes Mythic haste as amazing, or a must-have spell, is relying on this interpretation. There are some tables that rule that Mythic haste doesn't let you make a move action in addition to a full round action. In that case, Mythic haste stops being an amazing spell, and becomes a merely good one. Being able to move over a hundred feet in a single round is a nice ability, and being able to easily draw a potion or do any other move-action-activated ability is useful from time to time. Mythic spells are typically lackluster, and this one is reasonably useful at the very least.