First strike rewrites combat in two steps instead of one. The creature that has it deals its damage before anything without it gets to swing back, so a blocker without first strike has to survive that initial hit to land its own. In practice this means a 2/2 first striker beats a 3/3 vanilla in combat, and a 1/1 with first strike and a buff effect behind it stonewalls creatures several times its size. The keyword turns power-versus-toughness math from a single subtraction into a sequence, and the side without first strike loses any creature whose toughness the first striker can match.
What it actually does at the table is set the floor on attacks and blocks. A first striker on defense makes the red player think twice about sending in two-power creatures, and on offense it pressures opposing blockers that would otherwise trade. It pairs viciously with anything that pumps power, with deathtouch (the pre-damage kill skips the retaliation entirely), and with lifelink on a stable body, since the life arrives before the counterpunch. Pump-based aggressive decks and small-creature white strategies are its natural home.
Its ceiling is the size of the creature carrying it. First strike on a 1/1 deters tokens and not much else; reach and trample do nothing to help it against a five-power flier. And once both sides have first strike, or the blocker has more toughness than the attacker has power, the keyword stops mattering at all. It is a tempo edge in a narrow band of combat, not a substitute for a real threat.