The man is throwing a toy to a brown and black dog in the park.
A man and a dog are at a park.
The man and dog being "at a park," implies that they are, "in the park."
Sentence 2 is less descriptive. if the man and the dog a are playing with a toy in the park, then they are in the park.
A man and a dog in a park can be playing with a toy, as dogs like to play.