Counterspell gives blue decks a simple, broad answer to a spell on the stack. For two blue mana, it can stop a creature, planeswalker, artifact, enchantment, instant, or sorcery before that spell resolves, making it useful when your deck needs one interaction slot to cover many possible threats.
Countering a creature spell prevents its normal entry abilities from triggering because the creature never reaches the battlefield. Once the creature has resolved, however, Counterspell cannot remove it or counter its abilities. Pairing it with a card such as Cyclonic Rift gives a control deck options both before and after an opposing permanent resolves.
Talrand, Sky Summoner rewards the instant cast with a Drake, turning reactive play into a growing board. A draw engine such as Rhystic Study can help keep enough cards available to continue trading answers for opposing threats. These pairings suit decks that want to spend mana carefully while building an advantage over several turns.
The main constraint is the double-blue cost. Ancient Tomb or Sol Ring may provide abundant mana but cannot supply either blue symbol, so your land mix needs to support holding up two blue early. Counterspell also cannot answer a land play, an activated ability, or a spell that cannot be countered. Save it for something that materially threatens your position, and consider whether using your mana on development is more valuable when opponents are making low-impact plays.
