Grass resists Water, Electric and Ground, three very used types.
Unlike most other types, Grass isn't all-out offensive (see Fire or Fighting). Some are, but most use powders, Leech Seed, stalling and annoying tactics to win. And despite their many weaknesses (Psychic/Grass has the most weaknesses in the game but both Exeggutor and Celebi are really good) they are mostly good Pokemon. Gotta love Sleep Powder and Spore.
Their second typings (if any) also help their diversity. Victreebel can hit with STAB Swords Danced Sludge Bomb, Jumpluff uses it's immunity to Earthquake to switch in and then Encore, Celebi makes use of it's Psychic powers and Recover and the like, Ludicolo's Water typing negates weaknesses - and it can speed itself up in the rain. It can be the second typing that helps a Grasser the most - but Grass is by no means a bad type.