The Grappled condition only reduces speed to 0.
The grappled condition states:
- A grappled creature's speed becomes 0, and it can't benefit from any bonus to its speed.
The other two bullet points are the end conditions for Grappled. So nothing changes for a creature attacking a grappled creature. I quite often have seen players get Grappled mixed up with the Restrained condition, which does have some additional effects:
- A restrained creature's speed becomes 0, and it can't benefit from any bonus to its speed.
- Attack rolls against the creature have advantage, and the creature's attack rolls have disadvantage.
- The creature has disadvantage on Dexterity saving throws.
Restrained is a significantly more debilitating condition than Grappled. The idea here is that with grappling, two combatants are locked into position, but are otherwise still able to fight. With Restrained however, the victim's use of their limbs is actually impaired in some way.
The Grappler feat improves a character's grappling abilities:
- You have advantage on attack rolls against a creature you are grappling.
- You can use your action to try to pin a creature grappled by you. To do so, make another grapple check. If you succeed, you and the creature are both restrained until the grapple ends.
With the Grappler feat, you can actually impose the Restrained condition on a creature you have grappled, which would then allow an ally to attack the grappled creature with advantage.