Thats intentional, because Pendulum Monster Cards can be Summoned as monsters OR activated as Spell Cards, and they do different things depending on which way you play them
Goblin Spymaster can speed up a game like no other
Konamis official blog clarifies some further details on Speed Dueling, as well as the identities of those variant cards
For other's it about imposing your will on the game, whether it be controlling the board state via mass destruction, pinpoint removal spells, or (heaven forbid) mana denial/destruction
SR Super Rare UR Ultra Rare SE Secret Rare UL Ultimate Rare PS Platinum Secret Rare CR COLLECTOR'S RARE QCSE Quarter Century Secret Rare Underworld Goddess of the Closed World X LIGHT Link 5 [ FiendLinkEffect ] ATK 3000 DEF - 4+ Effect Monsters You can also use 1 monster your opponent controls as material to Link Summon this card
It's a very strong card in the modern game where combos can start with just one card is common