If your opponent has a stronger monster on the field, you can ram into it with "Alien Warrior", giving it 2 A-Counters, destroy it with "Mysterious Triangle", summoning another 4 star Alien like "Alien Shocktrooper", attack directly with your new monster, then activate this card to attack again with a new monster.
Aliens specialize in taking control of your opponent's monsters, through cards like "Brainwashing Beam" and "Alien Hypno". You can use this card to turn that into an even larger Alien swarm with your opponent paying the price for it.
This card can be used to "save" your own monsters if they are being targeted by an opponent's card effect, such as "Sakuretsu Armor", by sending it to the Graveyard and summoning another in its place.
It can also be used simply to make sure you have the right Alien on the field that you need. If you summon an "Alien Hypno" with this card, you are free to use up your Normal Summon for the next turn to use its effect.
It's good to memorize what Alien Monsters you have in your deck if you plan to use this, and keep track of which monsters have already been destroyed, so as to not suffer the 2000 damage.
If you don't mind the 2000 damage, you can use "Mass Hypnosis", take control of your opponent's monsters, and send them all to the Graveyard with this card. You won't get a monster out of it, but you will have cleared your opponent's field of monsters.