Creatures with the flash ability are extremely useful. You can play them to block, play them at the end of the opponent's turn so you can attack with them almost immediately and use their come into play effects as instant spells, but there are a couple minor weaknesses to the ability. The first of these is that creatures with flash cost more. This is fine when you use the ability, but when you cast them during your own turn(however rare that might be) they become expensive creatures. The second problem is that they are a limited resource. There are only a few and so often the ability you would most like as an instant isn't really available.
Scout's Warning is a rare instant from the future Sight expansion set of magic the gathering. Costing one white mana the card reads "The next creature card you play this turn can be played as though it had flash. Draw a card."
The effective meaning of this card is that so long as you have this any creature can have flash for a single white mana, and it doesn't even cost you a card since you get to draw another. And there are many creatures and effects that are very useful at instant speed.
Naturally one of the most common uses for this will to be to play a large blocking creature at instant speed. This like the card giant growth can make an opponent wary to ever attack because he can't be certain what you will do.
More useful though is instant speed come into play effects. Some of the most devastating instant effects are those that are typically sorceries. And at the top of that list is discard. Discard as a sorcery gives you opponent a turn to play the card he drew, but as an instant you can make him discard at the end of his draw phase meaning that unless he has an instant he can't use it. Not too bad early in the game but if he's on the draw it can be a free turn. Another are wrath of god effects. Using this on your opponent's turn isn't as big of an advantage, but it does leave your mana open to cast creatures so that you can get a step ahead on the new creature race and that can be important.
There are as many possibilities with this card as there are creatures in magic the gathering and as a one mana cantrip it is so inexpensive that you really can use it.