However, this creature removal enchantment can only target tapped creatures. While this sounds like a major drawback, because Seal Away has Flash, you can play it at instant speed. Here are three ways to play Seal Away effectively.
Play Seal Away in response to a creature declaring an attack
As long as a creature doesn't have vigilance, it needs to tap in order to declare an attack. This means that you can play Seal Away in response. The enchantment will exile the creature before any combat damage can happen. However, any abilities that activate as that creature declares an attack may still activate. Even still, Seal Away is still a nifty combat trick and useful creature removal.
Play Seal Away in response to a creature tapping for an ability
Since many creatures have activated abilities that require it to tap, Seal Away is a useful way to remove these types of creatures. While the tap ability will still activate and resolve, Seal Away will then take it off the battlefield for as long as the enchantment remains in play. It's a sneaky way to remove a creature that your opponent may not be expecting.
Play Seal Away as it re-enters the battlefield after being on exile from another card effect, targeting another creature
One of the drawbacks of a card like Seal Away is that if the enchantment leaves the battlefield for any reason, the exiled creature can return to play. However, if Seal Away re-enters play after being exiled, it can target a new tapped creature. Since the creature that re-enters most often comes back into play untapped, it won't usually be a valid target. If Seal Away comes back into play and an opponent has no value target, the effect of seal away will fizzle and it will go to the graveyard.
Beyond its obvious uses during its time in Standard, is Seal Away good in eternal formats such as Modern or Commander? It's unlikely that it will see much play in Modern, as Path to Exile is a much superior card. Path to Exile is only one Mana, doesn't have to Target only tapped creatures, and already sees a ton of play in the format competitively.
On the other hand, in Commander, Seal Away is a budget replacement for Path to Exile. While it's a greatly inferior card, it's still a useable card. Also, being an Enchantment, cards with the Constellation ability that activates whenever you have an Enchantment enter the battlefield will benefit from Seal Away.
Seal Away vs Ixalan’s Binding vs Cast Out
In Standard, Ixalan’s Binding and Cast Out are very similar cards. Binding can target any nonland permanent your opponent controls, but it doesn't have flash and it costs 4 Mana. The upside is that your opponent can't cast spells with the exiled spell's name - which can be relevant. Similarly, Cast Out costs 4 Mana, but it also has Flash. It can also be cycled away to draw a card for a single White mana.
Seal Away is only good against creatures. But in some matchups against creature heavy decks, costing only two mana makes it a playable card. It's worth playing in the main deck if your deck is weak against creatures in general or larger creatures. But against some decks, it will be sent to the sideboard.
Seal Away is certainly a good card and it makes sense that it's been one of the best selling cards in the Dominaria set. While Standard is its best format, don't be surprised to see it pop up in the occasional Commander deck, too.